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 _roles_ 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, coelenterates, 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.
II. ARGUMENTS AGAINST THE DOCTRINE OF DETERMINANTS.
Weismann has united his doctrine of determinants with his assumption of a differentiating division. He conceives that every little group of cells in 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 represented in the egg and in the spermatozoon by a number of little particles--the biophores--and that these, joined in a system, form the determinants. The innumerable determinants, he thinks, are, so arranged in the germplasm, and are endowed with such powers, that, during the process of development, 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 many architecturally arranged determinants would be present as there were different spots and stripes in the fur, due to colour and length of the hairs.
This chain of ideas, made sharp and definite by Weismann, has recurred again and again in theoretical biological literature in a vague way. In my view, it rests upon a false use of the conception of causality, and upon a false implication given to the relation between the rudiment and the product of the rudiment, each mistake involving the other.
Because, if its development be not interfered with, a definite egg necessarily gives rise to a definite kind of animal, a complete identity between the rudiment and the product, between cause 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 organic perpetual motion. It has been overlooked that, in the course of the development, many other conditions must be fulfilled, as without them the product never would come from the rudiment.
That the same adults may come from the eggs depends upon the egg-cells, in the ordinary course of events, being in similar conditions of anabolism and katabolism, being affected by gravity, light, temperature, and so forth, in the same way. Thus, when we are attempting to grasp the fundamental nature of the course of organic development, we must not omit the part played by these factors.
We may dwell for a moment upon this weighty point, as its significance is commonly misunderstood.
The course of each organic development depends in the first place, upon the absorption and metamorphosis of matter. Inorganic matter perpetually is being turned into organic material to serve for the growth and development of the rudiments. Thus, what in one stage of the development is mere inorganic material, and an external condition 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 the rudiments and altering them, even though the alteration may be purely quantitative. From this follows the very simple inference that during the course of an organic development external matter is always being changed into internal matter, or that the rudiments are continually growing and changing at the expense of the surroundings.
Now, let one reflect that the egg and the adult are two terminal states of organised material, and that they are separated from each other by an almost inconceivably long series of connecting, intermediate states; consider that each stage of the development is the rudiment and the producer of the succeeding stage, of the stage that follows, as the consequence of it; consider that what was external in each antecedent 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 development, and the causes entering it during the course of development from the accession of external material in the various stages. As there can be no absolute identity between rudiment and product, it is erroneous to transmute the visible complexity of the final stage of the development into an invisible complexity of the first stage, as the old evolutionists did, and as the new evolutionists are attempting to do.
But there is another error in the doctrine of determinants. This is in intimate union with the error just discussed, and, to put it shortly, consists in attributing to a cell--and the egg and spermatozoon are cells--the possession of characters not peculiar to cells, but resulting from the co-operation of many cells.
The characters of an adult active organism, like a plant or an animal, are exceedingly numerous, most varied in their nature, and essentially different. Some characters depend upon the healthy co-operation of nearly all the parts 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 cells. Is it really possible that all these characters, so many and so heterogeneous, have special, material bearers in the germ, and that these bearers are either simple biophores or determinants--that is to say, groups of biophores?
I can conceive a cell as endowed only with the material bearers of such characters as really belong to a cell itself. Thus, a reproductive cell might have material particles as the rudiments for producing horn, 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 hairs, nerve-ganglia, muscles, and so forth, must be groups of cells, for only groups of cells, and not specially arranged groups of particles within a cell, are able to grow into hairs, spinal ganglia, or muscles.
In a short statement, made in 1892, I said: 'The mistake into which speculations upon the nature of organic development has led so many investigators is this: they reflect the characters of the adult upon the undivided egg, and so people 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 multiplies by division into numerous organisms like itself, and that, in each stage of the development, it is only by the mutual action of all these numerous elementary organisms that the development of the whole organism slowly proceeds.'
Weismann himself, in a discussion of the pangenes of De Vries, has partly shown that one cannot assume the existence in the cell of material particles that are the bearers of qualities foreign to the nature of a cell and transcending it. In reference to the attempt to explain zebra-striping by pangenes, he says (_Germplasm_, English edition, p. 16): 'There can be no "zebra-pangenes," because the striping of a zebra is not a cell character. 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 to the peculiar arrangement of the cells. The same argument would apply to almost all the obvious "characters" of the species, genus, family, and so on. For instance, the size, structure, veining, and shape of leaves, the characteristic and often absolutely constant patches of colour on the petals of flowers, such as orchids, may be referred to similar causes. These qualities can only arise by the regular co-operation of many cells.'
Notwithstanding so correct a declaration, Weismann himself, in his doctrine of determinants, has fallen into the error he himself has exposed. To represent characters of the adult due to groups of cells and organisms, he imagines in the egg-cell, not simple particles like pangenes, but architecturally arranged groups of particles, determinants.
No real change has been made. Conditions are reflected upon the cell that in their real nature surpass its possibilities. With right and reason one may adduce, against his own determinants, what Weismann has said about pangenes, for exactly the same reasons: 'There cannot be zebra-determinants or serration-determinants, because zebra-striping, like the serrated edge of a leaf, is no cell character.'
The error in Weismann's doctrine of determinants may be made clearer by an analogy.
The human state may be conceived as a high and compound organism that, by the union of many individuals, and by their division into classes with different functions, has developed into a form always becoming more complicated. To carry out our comparison better, let us assume that all the individuals 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 organisations for protection, for tilling the soil, for trade, for government, and for education, must be explained causally from the characters of the first pair, which we take as the human rudiment, and from the outer conditions under which that pair and the generations that arose from it had to live.
As the state develops, urban and district communities, unions for husbandry and manufactures, colleges of physicians, parliaments, ministries, armies, and so forth, appear. All this visible complexity depends upon individuals associated for definite purposes and specialised in different directions.
It would certainly not occur to anyone to explain the growth of this complexity 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 complex characters of the human state by a system of architecturally arranged particles stored within the first pair. The organisations arising from the co-operation of many men are something new, and cannot be regarded as present in the organizations of one man. No doubt they depend, in the last resort, upon human nature, but by no means in this crude, mechanical fashion.
But what applies to the causal relations between the state-organism and men applies also, _ceteris paribus_, to the explanation of the causal relations between the rudiments 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, as that implies a fundamentally erroneous assumption. It refers organizations that depend upon cell-communities to organizations of material particles within a cell.
'To understand inheritance,' says Naegeli, with truth, 'we require not an independent, special symbol for every difference resulting from time, space, and quality, but a substance that, by the linking of the limited number of elements in it, can exhibit every possible combination of differences, and that by permutation can pass into another combination of differences.'
This standpoint is clearer when interpreted in terms of cells. The hereditary masses contained in the egg and spermatozoon can be composed only of such particles as are the bearers of cell-characters. Every compound organism can inherit characters only in the form of cell-characters. The innumerable, 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 many cells, and, for this reason, cannot be carried into the hereditary mass of any cell by material bearers. They are secondary formations, that can arise only after the multiplication of cells, and from the varied combination of cell-characters that accompanies the multiplication of cells.
In the foregoing pages I have attempted to prove the untenability of the doctrine of determinants from general considerations. I shall now attempt the same by analysis of a concrete case. The frog's egg may serve for this.
It is a familiar object, frequently studied. Consider its mode of division, and the formation of the blastula, gastrula, and germinal layers.
In cleavage the nucleus plays the chief part, and thus has been accepted as the bearer of the hereditary mass. But no single, special determinant gives the impulse for cleavage; rather, the co-operation of all the particles that are essential to the nature 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 condition to divide. This condition itself appears the necessary result of many different processes of nutrition and growth, as the result of complicated chemical processes that run their course within the separate, elementary, vital units of the nucleus.
The multiplication of the nucleus into two, four, and eight daughter-nuclei, and so forth, gives the impulse for the breaking up of the yolk into a corresponding number of cells. In that process the direction of the cleavage-planes, 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 within the nucleus. For all these phenomena, which are characteristic in the cleavage of the frog's egg, as well as in the cleavage of all other eggs, are determined directly by the qualities of the yolk surrounding the nucleus.
In several publications I have shown clearly that the external form of an egg and the arrangement of its contents, according to the different specific gravities of the component particles, determine the position of 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 yolk, upon the cleavage of the yolk into a portion richer in protoplasm and a portion poorer in protoplasm, and upon the differences in the bulk of protoplasm that in this way reaches each of the first-formed cells.
In many cases it has been shown that there is a constant relation between the first three cleavage-planes of the egg and the long axis of the animal that arises from the egg. Weismann and Roux make this a proof that, 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; that, similarly, the first transverse and horizontal cleavage-planes divide the protoplasm of the egg into pieces predetermined for the formation of the anterior and posterior, dorsal and ventral, parts of the embryo.
But I think I have shown beyond possibility of doubt that these events are due not to the existence of special, mysteriously working groups of determinants within the nucleus, but merely to the specific shape of the whole 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 influence upon the formation of the shape of the embryo. And so, in a recently published work, I stated that the growing embryo, especially in its early stages, must conform in many ways to the shape of the fertilised egg.
Thus, to bear out what I have been saying by actual examples, the distribution of the actual particles of the fertilised egg must correspond to the disposition of the bulk of material in the blastosphere; for, in the breaking up into cells, the spacial arrangement of the substances of different 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 cases the more or less complete segregation of the yolk and gravity, which causes a separation of the contents of the egg according to the weights of the particles, are agencies determining the particular kind of development. It is no case of special groups of determinants within the nucleus.
Thus, an oval and an elongate egg produce respectively an oval and an elongate blastosphere. The blastosphere determines the orientation of the gastrula, and so forth. In fact, the original distribution of mass in the material of the egg is carried directly on to the following stages of development (oval eggs of triton, insects, etc.).
So, finally, in many eggs, where, in addition to a polar differentiation, there is also a bilateral symmetry in the distribution of substances of different specific gravities and of different physiological value, the resulting blastospheres, from the reasons given above, assume a bilaterally symmetrical form.
Although, then, in eggs with polar differentiation, which have either one axis longer or are bilaterally symmetrical, under normal conditions the planes of the first two segmentations may correspond to the principal axes of the future 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. It is in this way that there are to be explained the investigations made by Van Beneden and Julin upon the eggs of ascidians, by Wilson upon the egg of _Nereis_, by Roux upon the egg of _Rana esculenta_, and by me on the egg of _Triton_.
As it fails with the process of cleavage, so Weismann's doctrine of determinants fails when we analyse the formation of the blastosphere, the gastrula, and the germinal layers.
The formation of the blastosphere seems to me to be due to the co-operation of the following processes:
(1) In the division of the egg-cell cavities arise between the four, eight, and sixteen pieces, and thus the whole contents of the egg become arranged more loosely. (2) The more the cells multiply by division and become smaller 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 constantly growing central cavity is formed _pari passu_ with the approximation of the superficial cells, and this probably also brings with it an increase of the internal pressure, and a wider curvature of the wall of the sphere.