The Biological Problem of To-day

A corner-stone of Weismann's theory is his assumption of nuclear divisions which are differentiating. Proof of this fundamental assumption may be sought in vain in Weismann's writings. Instead of that, a series of abstract arguments are brought forward in favour of it. Thus on p.

31 (of the English translation) Weismann treats the chromatin in the nucleus of the fertilised egg as the substance which accomplishes inheritance, and he denotes all the nuclei of the organism arising from the nucleus of the egg by divisions as the chromatin-tree, and then goes on to ask whether or no the pieces of hereditary material that make up the chromatin-tree of an organism are like each other or different. 'It can easily be shown,' the answer runs, 'that the latter must be the case.' For 'the chromatin is in a condition to impress the specific character on the cell in the nucleus of which it is contained. As the thousands of cells which constitute an organism possess very different properties, the chromatin which controls them cannot be uniform; it must be different in each kind of cell.'

Moreover, on p. 45 (of the English edition), 'The fact itself' (the capacity on the part of the idioplasm for regular and spontaneous change) 'is beyond doubt. When once it is established that the morphoplasm of each cell is controlled, and its character decided, by the idioplasm of the nucleus, the regular changes occurring in the egg-cell, and the products of its division in each embryogeny, must then be referred to the corresponding changes of the idioplasm.'

Finally, on p. 205 (of the English edition), 'The cells of the segmenting ovum are completely dissimilar as regards their hereditary value, although they are all young and embryonic, and are not infrequently quite similar in appearance. It therefore seems to me to follow from this, as a logical necessity, that the hereditary substance of the egg-cell, which contains all the hereditary tendencies of the species, does not transmit them in toto to the segmentation cells, but separates them into various combinations, and transmits them in groups to the cells. I have taken account of these facts in considering the regular distribution of the determinants of the germplasm, and the conversion of the latter into the idioplasm of the cells in the different stages of ontogeny.'

In the different propositions I have quoted, we have to deal with what is merely a fallacy in rhetorical disguise. For, from the premiss that the chromatin has the power of impressing specific character upon the protoplasm of the cell, it by no means follows that two cells, distinguishable by the nature of their plasma-products, must therefore contain different kinds of protoplasm. There are other possibilities to be reckoned with. Weismann himself knows that there is no logical necessity for the conclusion, for he himself suggests another possibility in the following: 'If we wished to assume that the whole of the determinants of the germplasm are supplied to all the cells of the entogeny, we should have to suppose that differentiation of the body is due to all the determinants except one particular one remaining dormant in a regular order, and that, apart from special adaptations, only one determinant reaches the cell, viz., that which has to control it. If, however, we do make the assumption,' etc. (p. 63, English edition).

Here, then, Weismann himself points out that what in other places he has attempted to represent as a necessary conclusion is but one of two alternatives.

Not only does he grant the possibility of the alternative, but uses it himself in explanation of the phenomena of reproduction and development. He attributes to certain series of cells, in addition to the active rudiments controlling the normal characters of their protoplasm, the possession of numerous latent rudiments which become active when opportunity presents itself.

This non sequitur in his argument Weismann excuses with the remark that the presence of latent rudiments in special cases 'depends, as I believe, upon special adaptations, and is not primitive, at any rate not in higher animals and plants. Why should Nature, who always manages with economy, indulge in the luxury of always providing all the cells of the body with the whole of the determinants of the germplasm, if a single kind of them is sufficient? Such an arrangement will presumably have occurred only in cases where it serves definite purposes' (p. 63, English edition). Here, again, is a rhetorical flourish instead of a proof.

But the dilemma which we are examining is not yet at an end. Supposing for the moment that we accept the assumption that different character in cells implies different character in their nuclear matter, we have at once a new and important decision to make. Does the nuclear matter in the different cells, that has arisen by division from the nuclear matter of the egg-cell, become unlike by the process of division itself? or is it only after the division that it becomes different, and in consequence of the action of outer forces upon the nuclei?

Weismann decides boldly-but again without bringing forward proof-in favour of the former interpretation. 'For the chromatin,' he remarks,[10] 'cannot become different in the cells of the fully formed organism; the differences in the chromatin controlling the cells must begin with the development of the egg-cell and must increase as development proceeds; for otherwise the different products of the division of the egg-cell could not give rise to entirely different hereditary tendencies. This is, however, the case.' Weismann represents to himself that[11] 'the changes of the idioplasm depend on purely internal causes, which lie in the physical nature of the idioplasm. In obedience to these, a division of the nucleus accompanies each qualitative change in the idioplasm, in which process the different qualities are distributed between the two resulting halves of the chromatin rods.'

I shall proceed to show that this conception involves material difficulties and contradictions. It will be found that characters totally contradictory are ascribed to Weismann's idioplasm. On the one hand, it is credited with being a stable substance, possessing a coherent, complicated architecture; in the form of ancestral plasms it is supposed to be handed on, from one individual to another, unchanged through many generations; on the other hand there is ascribed to it a labile architecture, that allows a free and perpetual casting loose of rudiments, of such a kind that at each division there is caused a complete rearrangement and unequal division of these rudiments. In the one case, the inner forces produce a reciprocal, coherent bond between the numerous rudiments; in the other case, permit change of their position and relations to one another, and this not only once but in orderly, definite fashion, different in each of many successive divisions, so that the id comes to possess a completely altered architecture. 'Each id in every stage' (p. 77 of the English edition), has its definitely inherited architecture; its structure is a complex, but a perfectly definite one, which, originating in the id of germplasm, is transferred by regular changes to the subsequent idic stages. The structure exhibited in all these stages exists potentially in the architecture of the id of germplasm: to this architecture is due, not only the regular distribution of the determinants-that is to say the entire construction of the body from its primary form.'

Unfortunately, Weismann's hypothesis tells us nothing at all about these internal causes, that depend upon the physical nature of the idioplasm; that is to say, nothing at all about the causes which, working in a fashion so contradictory and astonishing, really produce the whole development.

In such a state of affairs it is better to turn to Nature herself; and to see whether or no the occurrence of differentiating division of the nucleus in the organic world is at all supported by the actual observations and investigations of those who study cells.

We shall examine (1) Unicellular organisms; (2) Lower multicellular organisms; (3) The phenomena of generation and regeneration; (4) alteration of structural growth due to external interferences (heteromorphosis); (5) A number of physiological indications that cells and tissues, in addition to their patent characters, contain latent characters which have reached them by doubling division, and which are representative of the species.

FIRST GROUP OF FACTS.-UNICELLULAR ORGANISMS.

Doubling division alone exists, or could exist, among unicellular organisms. The maintenance of the species depends upon this. Our belief that a species produces only its own species, that like begets only like, a belief that finds continual confirmation all through the study of systematic and embryological natural history, would disappear, were it possible that in the division of unicellular organisms the hereditary mass should be split into two unequal components and be bestowed unequally upon the daughter-cells. All research shows that unicellular fungi, alg?, infusoria, and so forth, in dividing, transmit specific characters so strongly and in detail so minute that their descendants, a million generations off, resemble them in every respect. No one has doubted the fact, and Weismann himself recognises that division, among unicellular organisms, is always doubling. The process of division, as such, appears never to be the means by which new species are called into existence among unicellular organisms. This is a fundamental proposition of cell-life, not to be doubted, and to be taken into account in the presentation of theories of heredity.

From the proposition that like begets only like the corollary by no means follows that mother- and daughter-cells must appear identical from the beginning. For the identity under consideration belongs only to the substance that is the bearer of specific characters, to the hereditary mass; besides that, a unicellular organism contains other substances, substances that change from time to time during its life. Many unicellular organisms pass through a regular series of developmental stages; the stages themselves being inherited, and following each other as infallibly as in the case of embryonic stages of higher animals.

The following will serve as examples of this. Podophrya gemmipara, an Acinetan, in the adult condition is attached by a long stalk, while the free end, at which is the mouth, is provided with suctorial tentacles. It reproduces by giving rise to many little buds, ciliated on the upper surface like free-swimming, hypotrichous infusoria. These, in appearance, are quite unlike the parent organism, and, after a vagrant existence in the water for some time, they attach themselves to a surface and produce a stalk, tentacles with suctorial pseudopodia, and so for the first time attain the maternal form.

Some Gregarines are large, jointed cells, divided into two pieces, a protomerite and a deutomerite; they are clad with a cuticle, under which lies a layer of muscular fibrils. After conjugation they encyst, the nucleus divides, and they break up into numerous peculiarly-shaped boat-like structures, (pseudonavicell?), which afterwards are set free as small, sickle-shaped embryos. These exceedingly small germ-cells afterwards develop into the very different, adult gregarine-cells.

If the characters of a species be associated with a hereditary mass, an actual substance that is handed on from the parent-cell to the offspring, it is clear that the infusoria-like vagrant young of the Acinetan, and the sickle-shaped embryos of the Gregarine possess it, although for some time they are quite unlike the parent organism. For at last they become an Acinetan or a Gregarine, exactly like the parent-cell from which they arose as embryos.

These circumstances, among unicellular organisms, are a weighty indication of the error of concluding, with Weismann, in the case of multicellular forms, that because cells are unlike in outward appearance, the hereditary mass, or, as I call it, the nuclear matter, within them is also unlike. Such an assumption would involve us in the greatest contradictions. For the supposition that the nucleus is the hereditary mass transmitting the characters of the species necessitates the conclusion, in the case of unicellular forms, that the hereditary mass remains in possession of all the rudiments of the cell while it passes through the various phases of its cycle of development. Otherwise, these phases would have to be acquired anew in each case. We must, therefore, represent the possibilities of exchange between the nucleus, in its capacity of bearer of the hereditary mass, and the protoplasm as being such that all the rudiments are not simultaneously in activity, but that some of them can remain latent for a time.

SECOND GROUP OF FACTS.-THE LOWER MULTI-CELLULAR ORGANISMS.

Although in the development of unicellular organisms the way by which like begets like is plain and intelligible enough, at least in the cases dealt with, it is different with multicellular organisms, which have reached a higher grade of development. Among them we have to do with a continuous process of development, in which the highly-differentiated, multicellular organism arises from an egg, and in turn gives rise to an egg, and so on in unending sequence. But the succeeding stages of the sequence are so exceedingly dissimilar in appearance that the question how one step of the series turns into the next, and, above all, the question how the similarity of organisms, separated by the egg-stage, can be transmitted through the egg-stage, form the deepest riddle offered to biological investigation. Here, in a completeness so wonderful that our intelligence can hardly apprehend it, are presented to us the qualities of the organic material of which cells are made. Here lies that dark secret into which the various theories of generation try to direct a beam of light, and seek to find out the direction in which explanation may be found.

An intermediate stage which may serve towards the explanation of these circumstances is presented by the lower multicellular organisms, such as threadlike alg?, fungi, and other simple creatures. In them cells arise by division from the egg or from the spore, and become united into an individual of a higher rank; these cells resemble one another so completely in appearance and in qualities that there can be as little doubt as in the case of unicellular organisms that they arose by doubling division.

It is certain, then, that there exist multicellular bodies, often consisting of many thousand cells, in which each part retains the qualities of the egg from which it arose by doubling division, and which, as that method implies, possess the rudiments of the whole of which each is a part.

In this category there naturally fall the multinucleated masses of protoplasm, sometimes highly organised, in which every nucleus, surrounded by a shell of protoplasm, is capable of reproducing the whole. I am thinking of the slime-fungi (Myxomycetes), with their peculiar formation of reproductive bodies; of the 'acellular plants,' which in some cases closely resemble multicellular species in their formation of leaf and root, and in their mode of growth, as, for instance, Caulerpa, the multinucleated Foraminifera and Radiolarians. For, according to our definition of the cell, a multinucleated organism potentially is a multicellular organism.

In this matter Weismann has assumed a position which leads to peculiar consequences. In his opinion, somatic cells and germ-cells were sharply distinct at their first appearance in evolution, and have remained so ever since. Transitional forms between them are nowhere to be found. It would be inconsistent with his theory of the germplasm had somatic cells contained germplasm as their idioplasm, even when the soma first came into existence. The phyletic origin of the somatic cells depended directly upon an unequal separation of the determinants contained in the germplasm. It would totally contradict his presentation if the somatic cells, even at their first origin in phylogeny, contained, in addition to their patent special qualities, the qualities common to the whole species in a latent condition.

Weismann's conception, therefore, implies that many of the lower multicellular organisms, having no somatic-cells, have no body. Take the closely-allied creatures Pandorina morum and Volvox globator, which Weismann himself brings forward as instances for his view; the latter has a body, the former has no body, as all its cells are able to serve for reproduction!

It is enough to have pointed out how contradictory are the interpretations in this matter. Enlarging upon them may be postponed at present, for we are concerned here not with the interpretation of individual cases, but with the principles involved in the question, and, therefore, we must pass on to show further reason for considering the existence of differentiating division highly improbable in the whole organic world.

THIRD GROUP OF FACTS.-THE PHENOMENA OF REPRODUCTION AND REGENERATION IN PLANTS AND ANIMALS.

The numerous phenomena of reproduction and regeneration appear to support the principle of doubling division-that is, of division in which the germinal substance is handed on to every part of the organism. Our review may be short, as the phenomena are matters of common knowledge.

In nearly all plants there exist, widely spread through the body, cells and cell-groups, which may be induced, by inner or outer influences, to give rise to a bud; the bud grows out into a shoot, ultimately producing flowers and genital products. Such happens both in parts of the plant above the ground and below it; in the latter case shoots arise from roots, and reproduce the species in the ordinary sexual fashion by bearing sexual products.

Thus, in the case of Funaria hygrometrica, a little moss, one may chop up the plant into tiny fragments, scatter these on damp earth, and see numerous moss-plants reproduced from the little groups of cells. By cutting little pieces from a willow, an experimenter may cause the production from slips of thousands of willow-trees, each with all the characters of the species, so that there must have been contained in each of the little pieces of tissue hereditary masses with the characters of the whole plant. Separate pieces of the leaves of many plants, as of the begonia, produce buds from which the whole plant may grow out.

An aptitude for reproduction like that in plants exists in many c?lenterates, worms, and tunicates. The polyps of hydroids and of bryozoa, the stolons of an ascidian (Clavellina lepadiformis), may give rise to buds in many places, and these grow up into the perfect hydroid, bryozoon, or ascidian. There must, then, be contained in the cells of the bud the germinal rudiments of the whole animal; this conclusion is more necessary as the individuals, produced from the buds, in due course bear sexual products.

Although in many higher animals and plants one sees that cells with the capacity for reproduction are limited to special areas, still, the capacity for regeneration often is very great. In a wonderful fashion animals will reproduce lost parts, sometimes of most complicated structure; just as a crystal, from which a corner has been chipped, will perfect itself again when brought into a solution of its own salt. A Hydra, from which the oral disc and tentacles have been cut off, a Nais deprived of its head or of its tail, a snail of which a tentacle with its terminal eye has been amputated, will reproduce the lost parts, sometimes in a very short time. The cells lying at the wounded spot begin to bud, producing a layer or lump, the cells of which resemble embryonic cells. From this embryonic mass of cells the lost organs and tissues arise-in Hydra, the oral disc with its tentacles; in Nais, the anterior end with its sense-organs and special groups of muscles; in the snail, the tentacle with its compound eye built up of elements so different as retinal-rods, pigment-cells, nerve-cells, lens, and so forth.

Even among vertebrates, in which the capacity for regeneration is the least, as in the restoration of the wounded parts small defects occur, lizards can reproduce a lost tail, tritons an amputated limb. From a bud of embryonic tissue there are elaborated in the one case whole vertebr?, with their muscles and tendons, and part of the spinal cord with its ganglia and nerves, in the other case, the numerous, differently-shaped, skeletal pieces of the hand or foot, with their appropriate muscles and nerves. The regeneration, moreover, is in strict conformity with the characters of the species concerned. Thus, from the facts of regeneration also, we must infer that cells in the vicinity of these casual wounds possess not only the special qualities which they possess as definite parts of a definite whole, but also the characters of the whole, and thus have the power of becoming buds, from which a complicated part of the body may be reproduced with the appropriate characters of the species.

FOURTH GROUP OF FACTS.-THE PHENOMENA OF HETEROMORPHOSIS.[12]

Of all the facts brought forward here, the phenomena of heteromorphosis perhaps bear most strongly in favour of my conception, and offer difficulties most irreconcilable with Weismann's theory.

Loeb uses the word 'heteromorphosis' to denote the ability possessed by organisms, under the stimulus of external forces, to produce organs on parts of the organism where such do not occur normally, or the power to replace lost parts by parts unsimilar to them in form and function. Regeneration is the reproduction of parts like those lost; heteromorphosis is the reproduction of parts unlike those lost.

Heteromorphoses are well known in plant physiology. When one cuts a slip from a willow, one may make the cut at the bottom of the slip and the cut at the top in any part of the willow-twig, yet still the lower end of the slip always produces rootlets, which are organs not normal to that part of the twig, while shoots will rise from the upper end. Moreover, either end of the slip may be made the root portion, and it is clear, therefore, that in every small area there are cell-groups present able to bear roots or shoots according to the determining conditions; and therefore that, in addition to the characters active at any time, there are present the germinal rudiments for shoots and roots, and, indeed, for the whole organism, since the shoots ultimately may bear genital products.

When the prothallus of a fern has developed normally, it is a flattened leaf-like structure which bears rootlets and male and female genital organs on the lower surface, i.e., on that turned from the light. But the experimenter may reverse this order, by artificially shading the upper surface, and strongly illuminating the lower surface.

Among the most interesting heteromorphoses are the galls, produced upon young plants when certain insects lay eggs on them, or when plant-lice irritate their tissues. From these abnormal stimuli there result active masses of cells which grow into organs of definite form and of complex structure. The galls, moreover, differ widely, in correspondence with the specific stimulus which was their initial cause, and with the specific substance, the stimulation of which resulted in the formation of a gall. By the action of different insects upon the same plant different galls are produced, and the galls of different plants may be distinguished systematically.

Blumenbach has already brought forward the existence of galls as an argument against preformation, holding them to be structures produced epigenetically, and, therefore, unrepresented by rudiments in the germ. I, also, consider them witnesses against Weismann's germplasm. They teach us that the cells of the plant-body may serve purposes quite different from those arranged for in the course of development; that cells modify their form in correspondence with novel conditions, and that they are forced into forming special structures, not by special determinants in the germ, but by external stimulants.

Galls exhibit yet another instructive kind of heteromorphosis.

Even the tissue of a leaf, turned into a gall by pathological conditions, retains the power of producing roots. Beyerinck has shown that galls of Salix purpurea, planted in moist earth, bear rootlets identical with those of the normal plant. As the roots of all woody plants are able to bear adventitious buds, De Vries thinks it probable that one could rear a whole willow-tree from a gall. That would imply that all the inheritable characters of the willow were contained even in the gall.

Loeb has produced heteromorphoses experimentally upon many lower animals, among which were Tubularia, Cerianthus, and Cione intestinalis.

In Tubularia mesembryanthemum, a hydroid polyp, there are stalk, root, and polyp-head. If one cut off the head, a new head will be formed in a few days, this being a case of regeneration. On the other hand, a heteromorphosis may be produced by modifying the experiment as follows: Both root and head must be cut from the stem; if the lopped piece of the stem be stuck in the sand of the aquarium by the end that bore the head, then the original aboral pole in a few days produces a head; if the lopped piece of stem be supported horizontally in the water, then each end of it produces a head.

In a Cerianthus membranaceus (Fig. 1), the body was opened by a cut some distance below the mouth, whereupon buds appeared on the lower edge of the slit, where the experimenter had prevented coalescent growth. These buds gave rise to inner and outer tentacles, and an oral disc was produced. Thus, artificially, an animal with two mouth-openings or two heads was produced; and, similarly, animals with a row of three or more heads may be produced.

Fig. 1.-Cerianthus membranaceus, in which a second oral aperture, surrounded by tentacles, has appeared as the result of an artificial slit. (After Loeb.)

Fig. 2.-Cione intestinalis, in which eye-specks resembling those surrounding the mouth have appeared in the neighbourhood of an artificial opening (a).

The third animal in which heteromorphosis was produced artificially was Cione intestinalis, a solitary ascidian, an animal more highly organized. In Cione (Fig. 2) the edges of the mouth-opening and of the cloaca are provided with numerous, simple eye-spots. Loeb, in a series of experiments, made incisions either into the inhalent or the exhalent tube; after a time eye-spots appeared round the edges of the cut; then the margin of the artificial oral opening grew out into a tube, even longer than the normal oral tube. 'If several incisions be made simultaneously at different places on the same animal, then several new tubes arise simultaneously.'

In the three cases, the cut surfaces, from which in Tubularia, a head, in Cerianthus, tentacles, and in Cione, eye-spots, took their origin, were made in different parts of the bodies and in different directions. Thus, again, we have an indication that there are present in most regions of the body cell-groups, which may give rise to complex organs in unnatural positions, and yet bearing the specific stamp.

These examples might easily be multiplied, and they serve to show that heteromorphosis in plants and animals implies the presence of numerous latent characters in cells and tissues, in addition to the characters proper to their normal position in the organism. These latent characters, under the impulse of stimulation from without, manifest themselves in abnormal formation of organs in abnormal situations. Save that they are in abnormal situation, the induced organs conform to the specific type in all respects, and indicate that all the cells of an organism contain, as the result of doubling division, the characters of germinal rudiments of the whole organism. On the other hand, heteromorphoses bear heavily against the doctrine of determinants. For it is impossible that, in the architecture of the germplasm, there can be provision, in the form of special determinants, for events so foreign to the natural course of development as these arbitrary, outer stimulants.

Heteromorphosis may be extended to include more than Loeb intended by reckoning under it artificially-produced modification of the early stages in the cleavage of the egg. I have in mind those experiments by Driesch, Wilson, and myself, in which the first cells of the embryonic history were induced to form parts of the embryo, to which in the normal course they would not have given rise. In these cases heteromorphosis begins from the first cleavage of the egg.

In an ingenious way Driesch compressed fertilised echinoderm eggs between glass plates, and so secured that the first sixteen cells were separated, not by alternate vertical and horizontal planes, as in the normal development, but only by vertical planes. In the resulting one-layered plate of cells the nuclei had relative positions quite different from the normal. As, notwithstanding this, the distorted eggs developed into normal plutei larv?, Driesch inferred that the cell material composing the earliest cells of echinoids is equivalent in all the cells, and that the cells may be pushed over one another like a heap of balls without disturbing in the slightest their capacity to develop. Such a permutation could be without injury to the developmental product only if one nucleus had the same qualities as another; that is to say, only if all the nuclei had arisen from the nucleus of the fertilized egg by doubling division.

Driesch is right to regard these experiments as incompatible with Weismann's theory. 'Only consider,' he remarks, 'how great a number of "supplemental hypotheses," how many "accessory determinants," would be required to make specification of the early stages of a development in which any nucleus may take the place of any other nucleus in the whole embryo.'

I myself have carried out similar experiments upon frogs' eggs-experiments with a double interest. The frog's egg has the poles different, and so has a definite orientation. Weismann and Roux themselves have used these objects to support their view that, at the first cleavage, nuclei with different qualities are formed.

On p. 64 of the English edition Weismann remarks: 'The fact that the right and left halves of the body can vary independently in bilaterally symmetrical animals points to the conclusion that all the determinants are present in pairs in the germplasm. As, moreover, in many of these animals-e.g., in the frog-the division of the ovum into the two first embryonic cells indicates a separation of the body into right and left halves, it follows that the id of germplasm itself possesses a bilateral structure, and that it also divides so as to give rise to the determinants of the right and left halves of the body. This illustration may be taken as a further proof of our view of the constant architecture of the germplasm.'

Roux[13] has based his mosaic theory upon experiments upon frogs' eggs. According to the theory, the first two segmentation spheres contain not only all the formative material for the right and left halves of the embryo respectively, but also the differentiating and elaborating forces for these, so that on the destruction of one cell, the other can give rise only to one lateral half of the embryo (hemiembryo lateralis). Roux, therefore, considers that by the first cleavage the nuclear material is broken up into unlike halves, by which the development of the corresponding cells is directed diversely, i.e., is determined in a specific fashion.

Fig. 3.-Diagrams of the Eggs of Frogs, which show how alteration of the cleavage process changes the mode in which the nuclear material is distributed. The nuclei indicated by the same numbers have the same descent in all the diagrams. All the eggs are viewed from the animal pole. A. Normally developing eggs. B. Eggs developing under compression by horizontal plates. C. Eggs developing under compression by vertical plates.

The error in these representations of Weismann and of Roux has been shown by varied experiments of my own. The eggs of frogs on the point of cleaving were flattened to a disc between vertically or horizontally placed glass-plates. In the first case they were flattened in the dorsoventral direction, i.e., the axis passing through the animal and vegetative pole was shortened; in the second case an axis at right angles to this was shortened. In both cases the course of cleavage, and the resulting distribution of the nuclei in the yolk, was artificially modified.

The diagrams A, B, C (Fig. 3) will make the results plain to the reader. A, represents the distribution of the nuclei after normal cleavage; B, the same, when the egg was pressed between horizontally-arranged parallel glass-plates; C, the same, where the flattening was produced by vertically-placed parallel glass-plates.[14]

The diagrams show the positions of the segmentation spheres and of the contained nuclei as seen from the animal pole. In stages where two layers of cells as a result of division lay one above the other, the cells of the lower layer are distinguished in the figure by shading. In the three diagrams the nuclei are numbered so that the reader may know how far they are removed from the nuclei of the first two segmentation spheres. The numbers are further exhibited in the following two genealogical trees:

In the three diagrams the nuclei with the same numbers have the same rank in descent, and therefore, according to the theory of Roux and Weismann, have the same qualities, while the nuclei with unlike numbers differ in qualities.

Let us now notice how the nuclei in the three processes of division, of which two are abnormal, are placed in the mass of the egg.

After the first division, the nuclei are alike in all three cases; after the second difference appears. In A1 and B1 nuclei 3 and 5 lie to the left; 4 and 6 to the right of the second cleavage-plane, which, according to Roux's hypothesis, corresponds to the median-plane of the future embryo; while in C they are forced into two layers, one above the other, nuclei 4 and 6 being dorsal, 3 and 5 ventral.

In the third cycle of division there is no agreement between the three cases.

In the diagrams A2 and B2 the nuclei still lie similarly to the right and left of the middle line; but in A2 they are arranged in two layers, in B2 in a single layer. The nuclei 8, 10, 12, and 14, which compose the upper layer in A2, form the middle of the disc in B2; and 7 and 9, 11 and 13, the ventral nuclei of A2, occupy the ends of the single-layered disc of B2, being closely pressed against each other.

In the diagram C2 there is actually no median-plane after the third cycle of division. The nuclei 9, 10, 14, 13, which in A and B form the right side of the mass, here form a dorsal layer with nuclei 7, 8, 12, 11, forming a ventral layer. In the fourth cycle of division the nuclear matter is still more variously distributed through the mass, as may be seen from comparison of diagrams A3, B3, C3.

Although, under normal conditions, the multiplication and division of the nuclear material occurs in an almost invariable and definite fashion, the mere altering of the spherical form to a cylinder or to a disc produces a method of division completely different, so far as the nuclei are related to each other in a genealogical tree. In the one and the other method of division the nuclei are brought into relation with different regions of the protoplasmic mass, and are united with these regions to form cellular individuals.

I had quite enough reason for what I said in my essay: 'If the doctrine of Roux and Weismann be true, and the successive divisions by which nuclei arise really place different qualities in the nuclei-qualities according to which the masses of protoplasm surrounding them become different and definite parts of the embryo-what a pretty set of malformations must result from eggs in which the nuclear matter has been shuffled about so wantonly! As such malformations do not occur, it is plain that the doctrine is untenable.'

We reach the same conclusion from consideration of the interesting experiments made by Driesch and Wilson upon the early stages of segmentation of the egg. In the cases of an echinoid and of amphioxus (Fig. 4) they succeeded in shaking apart the first two and the first four cells that arose in division of the egg; and they traced the subsequent development of these separated segmentation spheres.

Fig. 4.-Normal and Fractional Gastrul? Amphioxus.

(After Wilson.)

A Gastrula from a whole egg; B, C and D, gastrul? from single cells artificially separated, (B) from the two-celled stage, (C) from the four-celled, and (D) from the eight-celled stages of normal development.

From one of the first two segmentation spheres of an echinoid egg, Driesch was able to rear successive embryonic stages (Gastrula and Pluteus), which were normal in shape, but one-half the usual size. Wilson's results, obtained by shaking apart the segmentation spheres, were even more interesting, as they were performed upon amphioxus, a more highly-organized animal. He reared gastrul? and older embryos with notochord and nerve-tube, which were perfect and normal, except in size. They were one-half, one-quarter, or one-eighth of the usual size, according as they were reared from cells isolated from the two, four, or eight-celled stage of the segmenting egg.

Results which Chabry and I gained by destroying, by puncture, one of the first two segmentation spheres, assist the present argument. Although one-half of the mass had been destroyed, Chabry obtained, in the case of an ascidian, and I obtained, in the common frog, embryos with notochord and nerve-plate. These developed directly and normally, although, in the case of the frog, there was a slight defect at the ventral posterior part of the body, where the arrested protoplasmic mass came to lie.

All these experiments show that the first two (and in some cases the first four) results of division can assume a quite different bearing as regards their function in the mechanical building of the embryo, according to whether they remain bound with each other into a whole or are separated and develop by themselves. In the former case, each forms only one-half (in some cases only a fourth) of the whole. In the latter case, each by itself produces the whole. The half and the whole, then, of the first cleavage-cells are identical in real nature, and, according to the circumstances, can develop, now in this way, now in that.

Even if Weismann were to admit the correctness of these experiments, perhaps he would not consider that they contradicted his theory of the germplasm and the segregation of the hereditary mass, but would make a supplemental hypothesis, which, from the spirit of his theory, could be none other than this: each of the first cleavage-cells, in addition to its specific part of the hereditary mass, the part that controls its normal course of development, possesses an accessory idioplasm, an undivided fragment of the germplasm, left behind to be ready for unforeseen emergencies; this part takes command when, in consequence of violence, a separated part develops into the whole.

But such an assumption does not go far enough, if it be confined to the first cleavage-cells. By compression of the frog's egg, I have shown that the pole passing through the blastopore, which coincides with the chief axis of the future embryo, may assume different relations to the first segmentation-plane, sometimes coinciding with that, sometimes making a right or an acute angle with it. It is clear that in each of these cases the embryonal-cells take a different share in the formation of the regions of the body, and that they must be fore-endowed with the capacity of playing different parts.

The developmental history of double monsters enforces the same doctrine; such are common among the embryos of fish, and rather less common among chicks. From causes of which we are ignorant two, instead of one, gastrula stages may arise at separate regions of the germinal layer of the egg. According to the position of these two invaginations, which may be regarded as crystallisation-points for the formation of the future embryo, the cells of the germinal disc will be drawn into the process of development, and, falling into groups, will build up organs. In relation to this double gastrulation, there may arise, for instance, four instead of two primitive ears, eyes, and nasal organs; and these arise from cell-groups, the choice of which is determined by their relation to the position of the gastrula-invagination.

From various other experiments, conducted so as to distort the normal course of development, I have obtained parallel results.

Taking frogs' eggs immediately after fertilisation, I compressed them strongly between parallel, horizontally placed glass plates. I then inverted them, so that the vegetative pole came to lie uppermost. In spite of their unnatural relation to gravity, they developed further, and became abnormal, quite unsymmetrical embryos.

In another experiment, taking a triton's eggs after they had divided into two spheres, I surrounded them with a silk thread in the plane of the first cleavage, and tightened the thread until the embryo assumed the form of a sand-glass. The deformity of the resulting larv? was very different, and perhaps depended on the tightness of the constriction. Some became greatly elongated, and had developed so that the thread surrounded the dorsal nerve-cord. In other cases the dorsally-placed organs arose only from one-half of the sand-glass-shaped embryo, while the other half gave rise to the ventral part of the body. In this case the dorsal organs (nerve-tube and notochord) were doubled over like a snare, the head and tail ends, the mouth and the region of the anus, being bent in at the position of the constricting thread.

The important point is that in both the experiments, in the case of the frog and of the triton, the cell-material, separated at the first cleavage, was turned to a use quite different to its use in the formation of a normal embryo.

We may conclude with a very convincing proof. In the above-mentioned abnormal development of the frog's egg it happened that one edge of the blastopore, on account of its weight, was very much bent outwards. In consequence of this the cleft of the blastopore lay between the normal blastopore-lip and the everted border of the other lip. When the notochord and the nerve-plate appeared, as a result of this abnormal condition, they grew from a cell-material that was quite different to that which gives them origin in normal cases.[15]

In these cases Weismann cannot apply his accessory conception, the existence of supplementary idioplasm, only to the nuclei arising from the first division; he must extend it to the thousands of embryonic cells that arise by division up to the time for the appearance of the nerve-tube and notochord. The behaviour of these cells under fortuitously changed conditions shows them all to be endowed with the capacity of development in different directions.

FIFTH GROUP OF FACTS.-PHENOMENA OF VEGETATIVE AFFINITY.[16]

Many considerations, taken from the region of general physiology, support the view that all the cells of an individual, of any species, are alike, and are to be distinguished from one another only by the special development of one character.

Formerly, indeed, many biologists, relying upon the optical appearances presented in microscopical investigation, have been inclined to the view that the visible qualities of a tissue, as revealed by the microscope, were the only, or the chief, distinctive characters. For instance, by microscopical investigation one cannot distinguish the tendons, nerves, bones, and cartilages of a dog from the corresponding tissues in a horse. So far as their special use in the organism goes, one might interchange the corresponding parts in these two mammals. A tendon from the dog, if large enough, might be attached to the muscle of a horse, and would transmit the pull of the muscle on the bone just as well, and would completely satisfy the mechanical duties of the horse's tendon. The same might happen in the case of a bone, of a cartilage, or of a nerve-fibre.

As a matter of fact, the idea that parts of the tissues of different animals may serve to replace one another has been employed repeatedly in science, especially in the science of medicine. But I believe that our ideas are not yet clear upon the matter. The erroneous impression to which I have alluded has arisen because we do not bear in mind that each tissue, each part of an organ, each cell, possesses, in addition to its obvious characters, very many characters that are invisible to us. Such characters are inherent in the tissue-cells because these are parts of a definite organism. In consequence of their specific tissue characters, which are visible to us, we assign cells their place in histological classification; in contrast, we may denote the other characters as constitutional, or species, characters.

No doubt tissue cells are in the same case as genital cells. So far as microscopical characters go, egg cells and spermatozoa are wonderfully alike in all the mammalia; in many cases we could not distinguish between those of different animals. But, because they bear the specific characters, we cannot doubt but that they are as distinct as are the species, although invisibly to us.

The products of the sexual cells show us clearly enough that out of each kind of egg only its own species of organism can be developed. Certainly it is not so plain that, besides their visible microscopical characters, the tissues and organic parts are in possession of more general characters, identical in all the differently-specialised tissues of a single organism; but we may infer the existence of such latent characters, at least partly, from the results obtained, in the case of plants, by grafting, in the case of animals, by transplantation and transfusion.

In the case of plants one may graft a twig cut from one tree upon the stem or lower part of another tree of the same kind, and so bring about a firm and lasting union between the two. In a short time the corresponding tissues of the parts brought into connection quietly unite. Thus from two different individuals a single living organism may be produced artificially.

One would expect, therefore, that a twig and stem, chosen from two closely allied species, such as, for instance, the pear and the apple, would unite when the suitable tissues were put together. But this does not happen. Successful grafting depends far less on the conjunction of obviously appropriate parts than upon characters unrecognisable by us, such as deep-seated kinship between the parts, and the specific characters of their cells; while in the case of individuals of the same species two pieces will unite even if they are not brought together in appropriate conjunction, or when they belong to different parts of the organism, as, for instance, to the root and the leaf; yet in the absence of deep-seated kinship union will not take place.

Generally this kinship, which has been called vegetative affinity, depends, like sexual affinity, upon the degree of systematic relationship. It appears that the same condition of things occurs as when, in ordinary fertilisation, sexual cells from different varieties, or species, are united. In both cases it happens, on the average, that union is the more to be expected the more closely the plants concerned are akin, in a natural system of classification.

But in grafting, as in cross-fertilisation, unexpected exceptions to this rule occur. Relying upon these, Naegeli thought that the external distinguishing tokens do not always indicate correctly the intrinsic constitutional differences. Frequently union will not take place between plants most near akin in classification, most alike in external characters; while it will occur between plants most different in outward aspect and belonging to different genera or even families. In other words, external characters give no certain index to the degree of vegetative affinity or of sexual affinity between two kinds of plants.

As an example of this, V?chting, in his treatise upon transplantation of plant-tissues, takes the tribes of pear-trees. Grafting between these and apple-trees takes place only with difficulty, although the apple is a close kinsman and belongs to the same genus. On the other hand, most of them graft easily upon the quince, although that belongs to a different genus. In this case, also, there is no sexual affinity between the pollen and the ova. Hybrids are not formed between the pear and the apple.

It seems probable to me, although as yet I cannot get complete proof of it, that sexual and vegetative affinity, that is to say, the relationship between the egg-cell and the pollen of two species, and the relation between twig and stem, depend upon the same intrinsic qualities of that elementary organism the cell.

V?chting distinguishes as harmonic or disharmonic the modes of union between twig and stem, according to whether or no they reach the formation of functional unity. Among cases of disharmony there are several interesting gradations. Generally speaking, in the case of plants not adapted to each other, no attempt at union occurs, and the grafted twig speedily perishes; sometimes even the stem dies, as if it had been poisoned by the graft. In other cases the disharmony is not shown so strongly. The twig and the stem begin to unite, but, sooner or later, disturbances occur, and complete destruction results. According to V?chting, in the case of some Crucifer? the disturbances are as follows: the twig begins to form roots at its lower end, and these grow into the stem of the host. Through them the twig uses as food the juices and salts of the stem, refusing to unite with the stem so as to form a single individual. As V?chting says, this formation of roots simply is an attempt on the part of the twig to complete its own individuality. Instead of growing into corporate union with the stem, the twig attempts to become a parasite upon it. A further consequence often is, that the stem, too, begins to respond to the unadaptive stranger's influence. Thus, when V?chting grafted a Rhipsalis paradoxa on an Opuntia labouretiana, he found that round the roots of the graft the tissues of the host threw out a protective sheath of cork, or turned in places to a gelatinous mass.

In some cases experimenters have overcome disharmony between two species, A and B, by making use of a third species, C, with a vegetative affinity for both A and B. Thus, an intermediary between the two disharmonic forms is made, and by such an arrangement a single functional individual is produced from pieces of three different species. Thus, upon A, as stock, a shoot of C is grafted, while upon this shoot of C, as stock, a shoot of B in turn is grafted.

In the matter of these different grades of disharmony, a comparison may be made between sexual and vegetative affinities. In many cases the spermatozoa of one species will not impregnate the eggs of another species. In other cases, the alien spermatozoon may penetrate the egg and unite with its nucleus, making, however, an unsatisfactory combination in various degrees of infertility. Sometimes the fertilised egg divides only a few times and then dies; sometimes development proceeds to the stage of the blastula, the gastrula, or even further; but it then comes to an end, through intrinsic causes beyond our ken, and, finally, complete destruction follows.

Our acquaintance with what happens in transplantation of animal tissues is smaller than in the sphere of botany.

Long ago, Trembley attempted to cause, by grafting, the union of two pieces of hydroid polyps into a single individual. He divided, across their middles, two specimens of Hydra fusca, and then, in a watch-glass, applied the upper end of one to the lower end of the other. In one case he was rewarded by the occurrence of complete union; for, after a few days, on feeding the upper end with a worm, it was passed on into the lower end. Later on buds arose, both above and below the point of union. Trembley, however, was unable to graft on each other parts of different species, parts of the green hydra, Hydra viridis, upon the common hydra.

Transplantations of single tissues or organs have been made more often, and by several investigators. I shall mention only the older results of Ollier and M. Bert, and those made in 1893 by A. Schmitt and Beresowsky.

Ollier exposed the bone of an animal, and, carefully removing a part of the periosteum, planted it in the connective tissue under the skin in another part of the body. The consequences differed according as the transplanted tissue was imbedded in another animal of the same species, or of another species. In the first case the piece of periosteum grew, obtaining a supply of blood from vessels which grew out into it from the surrounding connective tissue in which it was embedded. In a short time lamell? of bone were formed by the layer of osteoblasts, so that a small plate of bone was formed under the skin. This, however, proved always but a temporary structure, for, being formed in an inappropriate spot, and, therefore, being functionless, it was soon reabsorbed. In the second case, however, in which the piece of periosteum was removed from the bone of a dog and planted in a cat, rabbit, goat, camel, or fowl (or vice versa), formation of bone did not occur; either the piece of periosteum was absorbed, or set up suppuration around it, or became enclosed in a cyst.

Paul Bert's experiments were the following. He removed pieces two or three centimetres long from the tails of white rats a few days old, skinned each piece, and planted it in the connective tissue under the skin of the same animal. In a few days circulation of blood was established in the pieces of the tails, by union with vessels from the connective tissue in which they were embedded. Muscles and nerves degenerated, but the other tissues, bones, cartilages, and connective tissue, grew vigorously, so that, in animals killed and examined a month after the operation, the pieces of tail, implanted when they were two or three centimetres long, had grown five to nine centimetres long.

The result was totally different when the transplantation was made from one species to another. When the tip of the tail of a Mus decumanus or a Mus rattus was transplanted to a squirrel, guinea-pig, rabbit, cat, dog (or vice versa), either extensive suppuration took place, and the piece was extruded, while sometimes the subject of the experiment died; or, after a less turbulent course, the alien piece was absorbed. The continuance of life and growth in the piece only took place when the two animals concerned were allied very closely. Thus success followed transplantation from Mus rattus to Mus decumanus (or vice versa), but not when it was from Mus sylvaticus to Mus rattus.

The recent experiments of A. Schmitt and Beresowsky lead to the same conclusion. The former succeeded in making pieces of living bone 'take' only when the transplantation was from one individual to another of the same species, or to another part of the same individual. Beresowsky transplanted pieces of frog's skin to the dog and the guinea-pig, and pieces of dog's skin to the guinea-pig, and always found that they died, or were thrust out as foreign bodies.

Precisely the same results follow transfusion of blood between animals of different species. There is complete agreement among investigators. When the blood is made to flow directly from the vessels of one animal to the vessels of an animal of a different species, as from the dog to rabbit, or from dog to sheep (or vice versa); or when it has been first freed from fibrin and then injected, the result is always the same. 'We have always found,' says Ponfick, summing up the results of the investigation, 'not only that blood of another species acts in strong doses as a poison, and in weaker or smaller doses is harmful, but that (and this seems to me my most important result) in every case the blood-corpuscles are destroyed almost completely, probably quite completely.' In a very few minutes, in the case of disharmonic kinds of blood, the red corpuscles degenerate, and the h?moglobin, becoming dissolved in the blood-plasma, soon appears in the urine. In the case of transfusion of similar blood between individuals of the same or of very closely related species, the h?moglobin does not appear in the urine except after very large doses; and Ponfick infers that the red blood-corpuscles, either all of them or most of them, remain unchanged in the new animal.

Landois has carried out transfusion between the remotest species, between different families of mammals, and between mammals, birds, and amphibia; from these he drew 'the inference, important for classification of animals, that those animals anatomically most nearly allied have their blood most closely alike.' In fact, 'the destruction of the foreign blood happens the more slowly the more nearly the animals are allied.' 'Thus, in doubtful cases, experiments on transfusion might settle degrees of relationship. Between individuals of the same species transfusion is a complete success; when the species are closely allied, the transfused blood disappears only very gradually, and large quantities may be transfused without harm. The further apart the animals may be, in a system of classification, the more violently the destruction of the foreign blood takes place, and the smaller is the quantity that can be endured in the vessels. Thus, in the extent to which blood transfusion may occur, I see a step towards the foundation of a Darwinian theory applied to cells.'

As yet, transplantations and transfusions between animals of different species have been considered with a view to their importance in surgery and in medicine, rather than from their purely physiological side. From the results given above, in which I believe, although there might be drawn from literature contradictory results-in which, however, I cannot feel confident-I am prepared to extend a conclusion to the animal kingdom that is better supported in botany: the conclusion that the cells and tissues possess, in addition to their definite microscopical characters, more general, intrinsic, specific characters, and, that one may speak of the vegetative affinities between tissues exactly as one speaks of the sexual affinities between reproductive cells.

SUMMARY OF THE CONCLUSIONS IN THE FIRST SECTION.

Summing up what has been said in the preceding pages, we find a large series of facts supporting our contention that cells multiply only by doubling division. First comes the fundamental circumstance that single-celled organisms exhibit only doubling division, as by that alone the permanence of species, which experience shows us to exist, is possible.

Secondly, some facts of reproduction were considered. The formation of germinal tissues, and, in the case of lower plants and animals, the occurrence of budding in almost any part of the body, are easily intelligible if every cell, like the egg-cell, has been formed by doubling division, and so contains the rudiments of all parts of the organism; and if thus, on the call of special conditions, every cell may become a germ-cell again.

Thirdly, great stress is to be laid on those experiments in which the process of development was interfered with at different stages, as these showed that the separate cells which arose by division were not predestined unalterably for a particular r?le, according to a predetermined plan (facts of regeneration and heteromorphosis).

Fourthly, the results of grafting, transplantation, and transfusion indicate that the cells and tissues of an organism possess, in addition to their patent microscopical characters, latent characters, which show themselves to be peculiar to the species.

How does Weismann attempt to reconcile his hypothesis of differentiating division with these facts? By the provision of different complementary hypotheses, which, as we have seen, amount to this, that he allows the set of rudiments which he had turned out by differentiating division of the cell to creep in again by a back-door. He accomplishes this by his idea that the germplasm may undergo, simultaneously, doubling and differentiating division. In these cases cell-division has a double aspect. According to Weismann, this is possible, because the egg contains many, sometimes as many as a hundred, ids, each of which is a combination representing the species. Weismann believes that in an egg, while it is preparing for its first division, the ids are arranged in two groups-an active army and a reserve army. By differentiating division the active army is broken up into the divisions, brigades, and regiments of determinants appropriate to the separate groups of cells, and so the course of the development is conducted according to a preconceived plan. On the other hand, the passive, reserve army multiplies by doubling division, and is sent along with definite parts of the active army as baggage in a fixed or inactive condition, so that it has no influence upon the normal course of development nor upon the characters of the cells (fixed germplasm, inactive, accessory idioplasm, bud-idioplasm).

In spite of this purely arbitrary, complementary hypothesis, the facts seem to me to show that Weismann assumed an untenable position when he attributed a reserve army of 'stable plasma' only to the sets of cells in which it was necessary to suppose its existence. The experiments of Driesch, Wilson, and myself show that a complete embryo may spring from a half or quarter of the egg, and that the set of nuclei first to arise may be shifted about in the egg like a heap of billiard-balls. In the face of such facts there seems nothing left for the theory of Weismann but to endow every cell with accessory germplasm to prepare it for unforeseen events. This, however, would sterilize the other part of the theory, the doctrine of determinants, and the mechanism of development dependent on a rigid architecture of the germplasm. Consider the confusion that would arise when the deploying of the active army was disarranged by external influences, now in one fashion, now in another, if the reserve army, with its store of latent rudiments, had to come to the help of the broken pieces. What would compel the rudiments disposed to activity according to the prearranged plan to become latent where they were no longer wanted? And what would stir into activity in the necessary places the originally quiescent rudiments of the reserve army? In fact, if the r?les of activity and quiescence are even once to be exchanged by the rudiments in the cell, what object is there in drawing a distinction so sharp between the two armies-the active army which carries out the process of development according to a plan prearranged in its minutest details, and a passive reserve army ordered into quiescence and carried as baggage?

But here we come upon the scarlet thread that continuously has traversed the theory of germplasm in all its changes. Weismann attaches the greatest importance to the distinction. The twofold nature of the process of development is a cardinal point in his theory, linked to his doctrine of immortality for unicellular organisms and germ-cells and mortality for somatic cells.

Between somatic cells and reproductive cells Weismann places a gulf that cannot be bridged. Only the reproductive cells contain real germplasm, and only these contain the conditions for maintaining the species, as they alone serve for the starting of new generations of development. The somatic cells, on the other hand, are endowed only with fragments of germplasm, and hence they are incapable of preserving the species, and are doomed to death. The reproductive cells, like unicellular organisms, are regarded as immortal, the somatic cells as mortal. According to Weismann, cells cannot pass from the one category to the other.

As I see Nature, this contrast has been artificially reasoned into her. From several reasons, I do not think that it exists. In the first place, I consider that the facts I have given show the hypothesis of a differentiating division of cells and germplasm to be not proven and arbitrary. Next, the reproductive-cells must be considered as much a part of the organism as any other tissue. Sometimes they form the greater part of the body, as in many parasites, and, like the other tissues, they are subject to death, unless the conditions necessary to their further development have occurred in time. But under such conditions other cell-complexes may have death averted from them, as, for instance, when a slip cut from a willow-tree is planted. Thirdly, the reproductive cells are derived from the egg-cell just in the same way as other tissue cells are derived from it. Like tissue cells in multicellular organisms, they arise by the specialisation of material separated from the egg-cell, and, like every other organ, attain the position assigned them in the plan of development in the course of the general metamorphosis of position that all the cells pass through. Often the sexual cells, like those of other tissues, appear at a distance of several cell-generations from the egg. The intervening generations are specially numerous in those animals and plants in which several sexless generations come between the sexual generations (e.g., many plants, c?lenterates, worms, tunicates).

I cannot agree to the existence (in Weismann's sense) of special germ-tracks. Naturally, I do not deny that the sexual cells arise from the egg after definite sequences of cell-divisions; but this happens in the case of all specialised cells, such as muscle, liver, kidney, and bone cells. The conception of special germ-tracks has no more significance than there would be in the conception of muscle, liver, kidney, and bone tracks. Though Weismann associates with germ-tracks the idea that germplasm travels along them, proof of this has yet to be brought forward.

Finally, a word about the meaning of 'immortal.' In a scientific work the word must be used in a philosophical sense. In calling a being immortal one implies both individuality and indivisibility. This, at least, was the view of the old philosophers, who have defined the idea of immortality. Thus says Leibnitz in his Theodice: 'I hold that the souls which one day become the souls of men existed already in the seed, that they have existed always in organised form in the ancestors, back to Adam-that is to say, to the beginning of things.'

In his doctrine of immortality, Weismann has not concerned himself with the two implications-individuality and indivisibility. He calls a unicellular organism immortal, simply because its life is preserved in the organisms arising from it by division. The immortality of the unicellular forms depends upon their divisibility, upon a property which, according to the philosophical use of the word, is incompatible with immortality. According to Weismann, one immortal organism gives rise to several immortal organisms, but, as these are subject to destruction by external agents, the separate individuals are mortal. The unicellular organism is not immortal in itself, but only in as much as it may give rise to other organisms. In this way Weismann comes in conflict with the idea of individuality, and is compelled to transform his conception. For he says 'that among unicellular organisms there are not individuals separated from each other in the sense of time, but that each living being is separated into parts so far as space is considered, but is continuous with its predecessors and successors, and is, in reality, a single individual from the point of view of time.' Consequently Weismann must take the same view of the germ-cells, which, according to his theory, are immortal in the same way as unicellular organisms, and, in the same sense, he must make a single individual of all the germ cells arising from a single germ cell, and, with them, of all the organisms developed out of them. Adam is immortal quite as much as unicellular organisms, for he survives in his successors.

In brief, Weismann assigns immortality not to the unicellular individual, but to the sum of all the individuals arising from it, all the individuals of the same species, living contemporaneously and successively-in fact, to the conception of a species.

In my view, what Weismann has tried to express by the word 'immortality' is no more than the continuity of the process of development. So he himself says in the course of a defence in which, however, he did not intend to give up the standpoint he had taken; he wishes to imply, by the immortality of unicellular organisms, only 'the deathless transformation of organic material,' or 'a transformation of organic material that always comes back to its original form again.'

Thus, Weismann himself really has implied that his distinction between immortal unicellar organisms, immortal germplasm, and mortal somatic cells, is a misconception. For the continuity of the process of development, or the mode of transformation of organic material, depends upon the continual formation and eventual destruction of newly-formed material, but in no way implies the continuous existence of the organised material in a state of organisation. From this point of view, the immortality of unicellular organisms and of the germplasm breaks down, and, above all, the artificial distinction between somatic cells and reproductive cells. For, in the latter, the organic process of development, with its transformation of organic material, also occurs.

Here I may give the conclusion of this division of my argument. Cells multiply only by doubling division. Between somatic cells and reproductive cells there is no strong contrast, no gulf that cannot be bridged. The continuity of the process of development depends upon the power of the cells to grow and to divide, and has already been set forth in the sayings-Omnis cellula e cellula, omnis nucleus e nucleo. Whatever novelty the doctrine of the continuity of the germplasm brings into this saying depends upon error, and is in contradiction to known natural facts.