PART I. (continued 2)

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 Jülin 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.

Now, is there any part of these processes that has to do with the breaking of the nuclear contents into groups of determinants with different qualities? By no means. The egg divides into many pieces, because such division is a general property of cells, and it is not associated with separate, special material bearers. The appearance of spaces between the cells, resulting from division, is due to forces some of which reside 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 division press against each other. These forces that press the cells together seem to increase, as the size of the cells diminishes, so that the cells approximate their lateral faces continually more closely. The secretion of fluid into the interior of the sphere and the resulting increase of the outer surface results from the characters of the whole wall, and cannot be explained by single, specially determined cells.

Finally, to take the case of the special kinds of blastospheres (e.g., of amphioxus, amphibia, reptiles, birds, and so forth), it has been 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-particles under the influence of gravity; that, in fact, the shapes are determined by the general gross conditions of the structure of the egg.

Plainly, the blastosphere cannot be pre-existing as a structure of particles in the fertilised nucleus; there cannot be blastosphere determinants. The conditions 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 contains the conditions for blastosphere formation. Here we have epigenesis—the appearance of a new formation, not the becoming visible of pre-existing complexity.

The conditions of gastrulation and of the formation of the germinal layers are similar. The invagination of the blastosphere comes about by the co-operation of all the cells of its wall, by local differences in the 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 upon changes in the entire nuclear contents, it follows that the growth of the blastosphere-wall, which is merely the sum of the growth of all the cells in it, cannot be determined by special groups of determinants.

As an attempt to explain gastrulation, the origin of the germinal layers and many other events of development, the doctrine of determinants has reversed cause and effect. Certain cells do not become invaginated into the segmentation cavity because they possess groups of determinants that impel them to the assumption of inner layer characters. 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 unlogical to speak of endoderm in the fashion of many textbooks and treatises on embryology, while the so-called endoderm cells still form part of the outer surface of the blastosphere, or even while they are still in process of formation by cleavage. For 'inner germinal layer' implies a condition of position which is created by the invagination.

In fact, it is impossible, in thinking of the gastrula as in thinking of the blastosphere, to conceive that in the egg, which is a simple cell, there can be preformed by material particles in the nucleus a condition which implies the existence of two layers of cells.

Thus analysis of a special case leads to the same conclusion as is reached by the general reasoning of the earlier part of this section.

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