The Biological Problem of To-day

the adult body possessed of definite character and of definite position in the body-in fact, every group of cells that is independently variable-is r

, they reach, at the right time, the right place for their expansion into cells. For instance, in the case of a mammal with parti-coloured fur, as ma

erature in a vague way. In my view, it rests upon a false use of the conception of causality, and upon a false impli

and consequence, has been assumed more or less consciously. The conception of the sequence has been as if an organism caused its own development in a closed system of forces, in a kind of or

ons of anabolism and katabolism, being affected by gravity, light, temperature, and so forth, in the same way. Thus, when we are a

his weighty point, as its signif

of the development of the rudiment, in the next stage is become a part of the rudiment. The food-yolk of an egg, for instance, like the oxygen of the atmosphere, appears, in its relation to the material of the rudiments, to be something supplied from outside, an external condition of the development; yet it is continually passing into

edent stage has entered the rudiment and become part of it in the succeeding stage. Then it will be understood that it is a logical error to assume that all the characters present in the last link of the chain of development have their determining causes in the first link of the chain. The mistake lies in this: in the failure to distinguish between the causes contained in the egg at the beginning of the develop

discussed, and, to put it shortly, consists in attributing to a cell-and the egg and spermatozoon are ce

of the body, or of a group of organs; others are peculiar to an organ, and may be referred to its shape, structure, position, function, and so forth. Others, again, depend upon individual cells, or even upon separate parts of ce

n, chitin, chondrin, ossein, pigment, or chlorophyll, or for nerve-fibrils, muscle-fibrils; but not for producing a hair, or a separate ganglion of the spinal cord or the biceps muscle. The rudiments for hair

ople that sphere of yolk with a system of tiny particles, corresponding to the parts of the adult, qualitatively and in spacial relations. But in this method of thinking, it is left out of count that the egg is an organism which mult

er. There may perhaps be black and white pangenes, whose presence causes the black or white colour of a cell; but the striping of a zebra does not depend on the development of these colours within a cell, but is due to the regular alternation of thousands of black and white cells arranged in stripes.' Again (p. 17), he says: 'The serrated margin of a leaf, for instance, cannot depend on the presence of "serration-pangenes," but is due

ror he himself has exposed. To represent characters of the adult due to groups of cells and organisms, he imagines

nd reason one may adduce, against his own determinants, what Weismann has said about pangenes, for exactly the same reasons: 'There ca

trine of determinants may b

uals united in the human state arose from a single pair. The single pair would be the rudiment of the whole state, and would bear the same significance in the development of the state, as the fertilised egg bears to the development of the adult. The characters of the state, its different organis

plexity in the developing state by the assumption that this secondary complexity was preformed as definite material particles present in the first pair, although the first pair is the rudiment of the whole. Much comment is unnecessary; everyone must feel that this attempt to explain the causal relations is on the wrong track, that it is perverse to try to explain the co

udiments in the egg and the organism to which the egg gives rise. For these an explanation cannot be expected on the lines of Weismann's doctrine of determinants,

resulting from time, space, and quality, but a substance that, by the linking of the limited number of elements in it, can

umerable, and endlessly variable, characters of plants and animals are of composite nature. They find their expression in differences of shape, structure, and function in the organs and tissues, and in the special methods in which these are interrelated. They depend upon the co-operation of

ions. I shall now attempt the same by analysis of a concrete case. The frog's egg may serve for this. It is a familiar ob

e of the nucleus. The chromosomes, which we may regard as independently growing and dividing units, must have doubled by assimilation of food material from the yolk; perhaps, also, the centrosome may have doubled in the same way before the nucleus is in a

anes, the relative positions and the different sizes of the cells exhibit, under normal conditions, the most marked regularity. But it may be shown directly that this regularity is not the result of special determinants lying w

the nucleus and of the successive planes of division. Similarly, the different sizes of the cells first formed and the unequal rate of division shown at the two poles of the egg depend upon the constitution of the

, in nuclear division, the nuclei that arise have different qualities; that the protoplasmic masses lying to the right and left of the median plane are set apart to build up the right and left halves of the embryo;

le egg and to the segregation of the yolk. It is self-evident that, as the body of the embryo builds itself up from the actual material of the egg, the way in which the material of the egg is disposed must be of great i

erent weights undergoes no change. Thus, amphibia, the eggs of which have the poles different in character, produce blastospheres the poles of which are unlike; while eggs, like those of the fowl, where the yolk does not divide, give rise to blastospheres with unsegmented yolk. In such cas

termines the orientation of the gastrula, and so forth. In fact, the original distribution of mass in the materi

metry in the distribution of substances of different specific gravities and of different physiological

uture embryo, the cause for this agreement lies in the structure of the egg, and is not to be looked for, as Roux and Weismann suppose, in differentiating processes of cleavage, undergone by the nuclei in their first divisions. I

ctrine of determinants fails when we analyse the formation

seems to me to be due to the co-op

aller in circumference, the more closely they apply their lateral surfaces to each other, especially at the outer surface of the whole, so assuming the arrangement of cell-epithelia. (3) By the secretion of fluid, a consta

eside within the single cells, some of which come from without. In especial, the assumption of a spherical shape-an assumption occurring also to a greater or less degree when the results of division leave each other-is caused by the yolk actively arranging itself round the two nuclei as centres of attraction. The attempt to become spherical is opposed by other forces, in accordance with which the cells resulting from

een already shown that these are produced by the shape of the egg, by the bulk of the yolk, and by the segregation of the yolk-particle

ions for the origin of the blastosphere come into existence only by the process of segmentation, and it is only by its capacity to divide that the egg contai

rates of growth in that wall, from dissimilarities in its curvature, from many causes which have not yet been sufficiently sought out and investigated. As cell division itself depends not upon special particles, but

rs. The reverse is the truth. Local conditions of growth cause the invagination of a set of the cells of the blastosphere-wall. This invaginated layer of cells, brought into a new position with regard to its environment, becomes the endoderm and receives the stimulus to assume the character appropriate to the new environment. It is unlogic

, to conceive that in the egg, which is a simple cell, there can be preformed by material

same conclusion as is reached by the general

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rmplasm, p

e, Gestaltung und Vererbung; Leipzig, 1893; Herbert Spencer, articles in Contem

ising from it, build up the organism. 3. The preservation of a constancy of bulk of the hereditary mass when fertilization occurs. 4. The isotropism of protoplasm. Following Pflüger, I mean by isotropism that the protoplasm of the egg does not contain local areas for the formation of different organs; but that, according to the conditions, any part of the protoplasm may be employed in the formation of any organ. Isotropism is merely the negation of His'

ish editi

ish editi

de Vries, Intracellulare Pangenesis (1889). H. Driesch, Entwicklungsmechamische Studien, i.-vi.; Zeitschrift f. wissenschaft, Zool., vol. liii.-lv. The same, Zur Theorie der thierischen Formbildung. Biol. Centralblatt, vol. xiii., 1

mosaic theory against Driesch and myself in (1) Ueber das entwicklungsmechanische Verm?gen jeder der beiden ersten Furchungszellen des Eies. Verhandl. der Anat. Gesellsch. der 6ten Versamml. in Wien, 1892. (2) Ueber Mosaikarbeit und neuere Entwicklungshypothesen. Anatomische Hefte von Merkel

s, he must abandon his mosaic theory, and this he has not done. I think in the present essay,

e vertical axis of the egg, which passes through t

r ersten Furchungszellen für die Organbildung des Embryo. Experimentelle Studien am Froschund

articular upon the writings of V?chting, Bert,

Pflanzenk?rper. Untersuchungen zur Phys

achtungen ueber die Bastarderz

e des os au moyen de la transplantation du périoste, etc. Journal de la

mentales sur les greffes osseuses

toire de la vitalité propre des tissus animaux. Annales

d die Ueberpflanzung (Transplantation). Handbuch d. Allge

ir à l'histoire d'un genre d

sfusion des Blute

nd experimenteller Beziehung. Arbeiten aus der ch

e zur Lehre von der Transfusion

von Hautstücken auf Thiere einer anderen Species. Ziegler's Beitr?ge z

RT

HEORY OF THE DEVELOP

position allows neither an explanation, nor even the beginning of an explanation, of the differences that arise among cells while the differentiation of the body occurs. 'Any explanation must in the first place account for this differentiation,' says Weismann (Germplasm, p. 224); 'that is to say, the diversity which always exists amongst these cells and groups of cells arising from the ovum must be referred to some definite principle. In fact, no one could even look at it as giving a partial solution of the problem, if differentia

some come to activity in each special case, and that the selection of those that become active is due to causes arising in the cour

not only the material but the motive force of the course of development. According to him, every cell must have become what it is, bec

m, but that none the less arise in orderly sequence throughout the course of the development. The causes we recognise are first, the continual change

causes, or motive forces that are provided by the action of surrounding things. None the less, it must be borne in mind that there is no sharp distinction between centrifugal and centripetal forces. On page 86 I showed h

but belonging to the individual, influence the idioplasm; every cell, indeed, as it grows and divides, takes up a definite place in the growing whole, and finds itself in a peculiar combination of conditions of organisation.' 'Not only influences within the individual affect the idioplasm, as that may be altered by external influences, and so may be forced to grow in a new di

ause-in other words, to show a definite stimulus producing a definite reaction upon the rudiment-this failure is not to be attributed to error in the principle. It is the natural result of the enor

is an organism that multiplies by division into numerous organisms like itself. I shall explain the gradual, progressive organisation of the whole organism as due to the influences upon each other of these numerous elementary organisms in eac

crease in bulk results in a cleavage into two, four, eight, and sixteen pieces, and so forth. The cleavage produces a constantly changing distribution in space of the nuclear material. The two, four, eight, and sixteen nuclei that arise by division diverge from each other and take up new positions inside the egg, in definite relations to each other. At first the particles of the egg

iplication causes not only changes of bulk, but also from time to time changes in quality; for each shape is bound up with definite

h the material to be employed (i.e., the span of a roof, the construction of a bridge depend upon the material in shape a

ars to be a function of gro

withstand the strain and pressure put upon it by external forces. All these, and many other factors less easy to conceive, must be delicately adjusted to one another. If in any direction a definite limit be exceeded, then either the structure will be destroyed by disintegration of the component parts, or a new shape will be assumed. The latter is the event in the case of a living substance capable of reaction. T

importance in its consequences. It may be stated in this saying: Growth always must be such as to produce the greatest possible ext

peculiar to the material of which it is composed. These particles, once crystallised, retain their position even when new layers are deposited on thei

ng it. Protoplasm cannot become fixed in any condition without being destroyed; it exhibits perpetual interchanges with the outer world; unceasing intak

s of their conditions of existence. Similarly, an extended membrane of cells or an epithelial layer cannot add indefinitely to its thickness, else would the cells furthest removed from the outside be injured in their relations to surrounding things. To satisfy its essential conditions, pr

s, or in leaf-shaped flattened organs. That they may suck up water and salts from the soil, the cells are arranged as a highly branched system of roots, covered with delicate hairs, and penetrating the soil in every direction. To inhale the carbonic acid from the air, and to be subjected to the influence of sunlight, the aerial part of the plant stretches out its branches towards the light, and becomes folded into the flat leaves, the structure of which reveals a suitability for assimilation. Thus the whole architecture of a plant is superficial and visible; internal differentiation into organs and tissues either is wanting, or, compared with animals, is very scanty. It is only in the higher plants that the internal fibro-vascular tissues appear; these serve a double purpose: they act as channels along which the sap passes, so bringing together the different materials absorbed by roots and leaves; and they have the mechanical function of strengthening the stem and branches. The different mode of nutrition of animals results in a totally different structural plan. Animal cells absorb material that is already organised, and that they may do so their cells are either quite naked, so affording an easy p

place in the internal surface. The specialisation of plants displays itself in organs externally visible-in leaves, twigs, flowers, and tendrils. Th

similar. There are required only the slightest variations of a few simple general laws. For these I may refer readers to a series of special investigations (S

tiation among the cells arising from the egg. Although each of these resembles the parent egg, from which they arose by doubling division, yet they differ from it in one point: they are no longer a whole, but have become the subordinate parts of a higher unity, that is, of a higher organism. A cell that is no longer a whole, but the part of

cting as wholes, and composed of several grades of structural parts, subordinate in function to the whole, and displaying only a limited division of capacities. And so the cell theory, according to which the cell was exalted unduly as the unit o

, it is merely one of the numerous expressions of the formative forces which reside in all matter, in the highest degree, however, in organic substance.' 'Essentially, every plant

rom the way in which V?chting set forth

displays the same functions as while it is a part of the organism. The cell while in the organism and the cell separated from the organism and self-sufficing, are quite different. We must regard the functions of a cell that is part of an organism, disregarding external influences, as determined by the whole organism, and only by the cell itself, in so far as that forms a greater or less part of the whole organism. When not part of an organism, the cell is independ

rious parts of the organism, and, on the other hand, to surrounding things. Naturally, such relations differ with the place or position occupied by cells in the whole organism, and in this way there come to be innumerable conditions making for diverging directions of development, for division of labour, and for dissimilar, histological differentiation. The part played by a cell, as V?chting puts it, will depend upon

e and function makes it easy to conduct

organs, according as it is kept in the light or in the dark. Similarly, taking a willow slip, roots may be

oot. The conditions that determine the mode in which these structures will develop are quite within the power of the experimenter. We have shown already and could show further, that he is able to determine the mode of growth by cutting, bending, tying in a horizontal position, and so forth: For such reasons, V?chting describes plants as masses of tissue, practically plastic, and which may be moulded at the discretion of the

placed vertically, in a short time new branches and new 'roots' spring from it. In this case, again, the position of the new growths is determined by the relation i

produce a hypertrophy of one of the lips of the blastopore. When fusion of the lips took place the normal lip united with the rim of the protruding hypertrophied lip. As a result of this the notochord and the nerve plate came to arise, not from the usu

r character and assume the qualities and aspect of the external skin, when, as in

organism, sometimes in one way, sometimes in another. One must note very carefully that Weismann's doctrine of determinants, according to which all tha

quotations from botanica

shoots will now produce roots. Precisely the same occurs with a piece of the root. In fact, it appears as if the idioplasm knew what parts of the plant were wanting, and what it must do to restore the integrity and vital capacity of the individual.' 'The idioplasm in the remaining pa

borated by Pflüger in his important treatise on T

ites in sim

ortioned tap-root, from which arise a due array of lateral roots. In normal conditions all these organs are in equilibrium. An apple-tree, growing on the line where tilled garden ground meets a lawn, grows more vigorously on the side towards the garden. If one of the roots of an apple-tree with three main roo

the Leaf: 'The fact that lateral buds do not develop while the axial bud is still growing vi

riety. When a pear-tree is grafted upon the quince, which is characterized by its dwarf-like growth, the vegetative growth of the pear is reduced exceedingly. It produces shorter and weaker shoots; all the dwarf varieties of the pear employed as wall fruits, or growing into the little pyramids spoken of in

a temperature lower than 7.5 degrees of frost, will survive 12.5 degrees if it has been grafted upon P. terebinthus. Moreover, when it is grown from a seedling, it may reach the age

that bore young buds gave rise to vegetative shoots when it was united with a young, still g

nd the increase in the nerves leads to corresponding increase in the nerve centres. The tendons of origin and of insertion, and the parts of the skeleton to which these are attached, must react to the increased size of the muscle by gr

hism and polymorphism. These seem to me to show how very different final results may grow from identi

ut the sexual dimorphism that occur

king characters affecting different parts of the body, and known as secondary sexual characters. In fact, the difference between the sexes may be so great that a systematic naturalist,

ry chamber of which it lives as a kind of parasite, and appears, so far as outward shape goes, more like a turbellarian than a gephyrean. None the less, male and female are alike not only while t

oth to males and to females are transmitted either by eggs or by spermatozoa. In parthenogenetic animals both male and female individuals appear at definite times from eggs produced without sexual commerce. Whether the male or the female forms be produced depends, not upon any difference in the germinal materia

tor, M. Maupas, on the determination of sex in Hyda

g the temperature at the time when the eggs are being formed in the germaria of the young females, the experimenter is able to determine whether t

he case of other five young females of the same brood, but kept in a cold chamber at a temperature of 14 to 15 degrees centigrade, 95 per cent. of females were produced. In another experiment, young animals were

bers, which produce on the same stem both male and female flowers, bear only male

etermination of sex depends upon fertilisation. Thus, among bees,

r instance, that among animals the early removal or destruction of the sexual organs hinders the development of the secondary sexual characters, or even may occasion the

ormous influence exerted by external forces upon correlated variation o

animals-first, because the matter has recently occasioned an important controversy between Herbert Spencer and Weismann; and, secondly, because the

sexless individuals known as neuters. These in certain cases are very di

organs are rudimentary, and parts of whose bodies-the stings, the wings, the hind legs, with

cture among the three or four forms are shown, frequently by considerable differences in size, by the presence and absence of wings, by differences in the sense-organs, the brain, and the structure of the head. In the common ant-Solenopsis fugax, for instance, as Weismann q

ee or four kinds of ants in the colony must be developed from eggs deposited by the females. In this Weismann finds the most conv

altered others; and the metamorphoses have at times been very important, demanding the variation of many parts of the body, inasmuch as many parts must adjust themselves so as to be in harmony with them.' 'None of these changes' (p. 318) 'can rest on the transmission of functional variations, as the workers do not at all, or only exceptionally, reproduce. They ca

hese attains to value-i.e., gives rise to the part of the body that is represented-and the others remain inactive.' This structure of the germplasm Weismann attributes to the operation of selection. 'For in the ant state' (loc. cit., p. 326) 'the barren individuals or organs are metamorphosed only

ue that Weismann holds his own explanation to be the only conceivable explanation. 'For there are only two possible a priori explanations of adaptations for the naturalist, namely, the transmissio

ly as Weismann suggests? Is there

different individuals in the insect states may be explained in a third way-in a way overlooked by Weismann. This third explanation is nothing more than the s

ialist upon the structure of ants, and Herbert Spencer, relying upon the investigations of several Englishmen, have sought to prove that the differences between the individuals in t

the hive in which the egg is placed, and on what food the embryo is reared. In the specially large cells, known as queens' chambers, and with specially nutritious diet,

s shown that termites have it in their power to alter the relative numbers of workers and soldiers, and to produce as many of the latter as may be required, a

ain nutritive materials by a greater growth of certain parts of the body, and a lesser growth of other parts. Workers' food stimulates growth in the jaws and brain, retards growth in the wings and sexual cells. Queens' food has the opposite action.' There is a correlation between retardation of

e of determinants. The observations of many careful persons, such as Charles Darwin, Emery, and others, show that in many species of ants the extreme types of individuals are connected by many intermediate forms. (Apud Emery, thi

e that while perfect females and the extreme types of workers have their different sets of determinants, the intermediate types of workers have not. Hence we are introduced to the strange conclusion that, besides the markedly distinguished sets of determinants, there must be, to produce these intermediate forms, many other sets slightly distinguished from one another-a score or more kinds of germplasm, in addition to the four chief kinds. Next comes an introduction to the still stranger conclusion, that these numerous kinds of germplasm producing these numerous intermediate forms are not simply needless, but injurious-produce forms not well fitted

inted out, Weismann makes his usual mistake. He incorporates in the rudiment what really are stimuli coming from external cond

neutral germinal material of an insect egg there is produced a male or female creature, or a worker or soldier (as this or that influence acts), the process is no other, and presents no greater difficulties, than when an experimenter, taking the young bud of a plant, according to the conditi

ition in its widest sense as a factor); (3) the interrelations of the parts of a whole (cells, tissues, and organs) to one another and to the whole (correlative development). There remains to be considered the extent to which the germinal material in the egg determines the course of development of the organism. Here, before all things, it must be insist

on the side turned towards the centre of light, while another becomes convex on that side, the cause can only lie in the internal structure of the organ. But it is just on such differences of structure that the great variety of reactions which the most different plant organs exhibit towards the same external influ

rotoplasm of different kinds of plants that the special shape of the whole root system depends: whether, for instance, the root system ramifies superficially o

kinds of organisms. These germinal substances must be possessed of an extraordinarily complex organisation, and must be able to react in specific fashion-that is to say,

agree with wha

egg as in the fowl, and the egg of a fowl differs as much from the egg of a frog as the fowl differs from the frog. Men, rodents, ruminants, invertebrates display more or less important and outwardly visible differences in constitution; so also

ception is quite different from their conception. For I can ascribe to the germinal substance only such characters as are appropriate to the true nature of a c

eory. 'For preformation,' he says, 'it is not necessary to imagine that the egg contains a miniature of the adult. If only, like Hertwig, one assumes to be p

ed initial plasm as the basis of the process of development. It may be called epigenetic, because the rudiments grow and become elaborated, from stage to stage, only in the presence of numerous external conditions and stimuli, be

cially in the relations that I conceive to exist between the rudiment and the adult, I

entiation, so the human community, a composite organism of a still high

their relations and their combinations, men in society have brought about a higher complexity than man, left by himself, ever would have been able

for the self-multiplying systems of units, always binding themselves into higher complexes, continually enter

he egg-cell into a man, and of men into a sta

may be carrie

other characters, and these come to play correspondingly different parts. The special differentiation undergone by any individual depends upon the special place he comes to occupy in the whole of which he is a part, not upon really different organ

f the whole in which the cells come to lie during the progress of the development, and according to the relations to the whole they come to assume. Thus, here they assume the characters of the external skin; there, they become gland-cells of the intestine;

e to their individual relations to the whole; the determining forces are not within the cells, as the doctrine of determinants supposes

less, consider how in a civilized state the apparently sovereign individual is conditioned in all his circumstances; how each change in the general state exercises an influence on the individual's disposition freedom of will, and method of life (dwelling, food, institutions, health); then reflect how much greater in the animal and the plant is the d

er to the relation of the specific nature of the rudime

colonies depend on the nature of bees; of ant colonies on the nature of ants; of the societies of men on the nature of men; indeed, in the latter case we see how they differ as t

nisation, and I assume that this is transformed into the adult product by epigenetic agencies. To a certain extent,

s no more than a description, in other words, of the visible events of development. To be more than this, it would be necessary to explain how in each case the biophores and determinants and ancestral plasms are constituted, and how they are arranged in the architecture of the germplasm so as to produce the development of the egg-cell in this or that fashion. It must, at the least, offer such possibilities as the structural formul? of chemists offer. But in the present stage of our knowled

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contains references to