The Evolution Theory, Vol. 1 of 2

by August Weismann

In ontogeny we see exactly the same thing. There are no two daughter-cells of a mother-cell which are exactly alike, and the differences between them, if they increase in the same direction, may lead in later descendants to entire differences of structure. Indeed the whole process of development depends on such an augmentation of the differences between two daughter-cells—on differences which proceed from within and are definitely pre-established. Here, again, the facts do not justify us in making a dogma of the proposition that it is a 'fundamental power' of every living being to maintain its species by producing replicas of itself. If we look at two directly successive cell-generations, we can hardly, it is true, in most cases, perceive any difference between them, just as in the generations of species; but if we compare the end of a long cell-lineage with the beginning, then the difference is marked, and we recognize that the difference is due to a gradual summing up of minute, invisible deviations. In my opinion these steps of difference cannot possibly depend merely on direct external influences; they proceed rather from the hereditary substance the cell receives from the ovum, which, therefore, in order to attain to such many-sided and far-reaching differentiation, must have undergone a frequently repeated splitting up of its qualities. That this splitting is not merely a variation to which the whole of the hereditary substance of the daughter-cells is uniformly subject, according to the influences dependent on their position in relation to other cells of the embryo, will be made clear from the case of the Ctenophora referred to in the next lecture. A scarcely less striking example is that of those animals in which the ova contain the primary constituents for only one sex, in which, in other words, there are 'male' ova and 'female' ova. This is the case, for instance, among Rotifers, and in plant-lice such as the vine-pest, Phylloxera. Here the eggs from which males develop are smaller than those which produce females. The primary constituents for both male and female are not, as in most animals, contained in the same ovum, to be liberated on one side or the other by influences unknown to us, but in each ovum there is only one of the two sets of primary constituents present, and in this case, therefore, the development of hermaphrodites, which not infrequently occur in other animals, would be impossible. But all these ova have been produced by one primitive reproductive cell, and consequently, at one of the divisions implied in the multiplication of this first cell, a separation of the male from the female primary constituents must have taken place, that is, a differential division of hereditary substance, for which no external and no intercellular influences can possibly account.

If there is, then, a differential division of the ids and with them of the whole idioplasm, the germ-plasm of the fertilized ovum must be broken up in the course of ontogeny into ever smaller groups of determinants. I conceive of this as happening in the following manner.

In many animals the fertilized ovum divides at the first segmentation into two cells, one of which gives rise predominantly to the outer, the other to the inner germinal layer, as in molluscs, for instance. Let us now assume that this is the case altogether, so that one of the first two blastomeres gives rise to the whole of the ectoderm, the other to the whole of the endoderm; we should here have a differential division, for the developmental import (the 'prospective' of Driesch) of the primitive ectoderm-cell is quite different from that of the primitive endoderm-cell, the former giving origin to the skin and the nervous system, with the sense organs, while the second gives rise to the alimentary canal, with the liver, &c. Through this step in segmentation, I conclude, the determinants of all the ectoderm-cells become separated from those of the endoderm-cells; the determinant architecture of the ids must be so constructed in such species that it can be segregated at the first egg-cleavage into ectodermal and endodermal groups of determinants. Such differential divisions will always occur in embryogenesis when it is necessary to divide a cell into two daughter-cells having dissimilar developmental import, and consequently they will continue to occur until the determinant architecture of the ids is completely analysed or segregated out into its different kinds of determinants, so that each cell ultimately contains only one kind of determinant, the one by which its own particular character is determined. This character of course consists not merely in its morphological structure and chemical content, but also in its collective physiological capacity, including its power of division and duration of life[20].

[20] Emery has lately called attention to another direct proof of the existence of differential cell- and nucleus-division. According to observations made by Giardina, in the water-beetle (Dytiscus), one primitive ovum-cell gives rise, through four successive divisions, to fifteen nutritive cells and one well-defined ovum-cell. But only half of the nuclear substance takes part in these divisions, the rest remains inactive in a condensed, cloudy condition. 'The meaning of the whole process is obviously that the germ-plasm mass as a whole is handed over to the ovum-cell, while the nutritive cells receive only the nuclear constituents which belong to them' (Biol. Centralbl., May 15, 1903).

But embryogenesis does not proceed by differential divisions alone, for integral divisions are often interpolated between them, always, for instance, when in a bilateral animal an embryonic cell has to produce by division into two a corresponding organ for the right and left sides of the body; for instance, in the division of the primitive genital cell into the rudiments of the right and left reproductive organs, or in the division of the primitive mesoderm-cell into the right and left initial mesoderm-cell, but also later on in the course of embryogenesis, when, for instance, the right or the left primitive reproductive cell multiplies into a large number of primitive germ-cells, or in the multiplication of the blood-cells, or of the epithelial cells of a particular region; in short, whenever mother and daughter-cells have the same developmental import, that is, when they are to become nothing more than they already are. In all such cases a similar group of determinants, or a similar single determinant, must in the nuclear division penetrate into each of the two daughter-cells.

It is in this way, it seems to me, that the determinants gain entrance into the cells they are to control, by a regulated splitting up of the ids into ever smaller groups of determinants, by a gradual analysis or segregation of the germ-plasm into the idioplasms of the different ontogenetic stages. When I first developed this idea I assumed that the splitting process would in all cases set in at the same time, namely, at the first division of the ovum. But since then, in the controversies excited by the theory, many facts have been brought to light which prove that the ova of the different animal groups behave differently, and that the splitting up of the aggregate of primary constituents may sometimes begin later—but I shall return to this later on.

If we accept the segregation hypothesis, which is similar in purport to that advanced by Roux as the' mosaic theory,' it must strike us as remarkable that the chromatin mass of the nucleus does not become notably smaller in the course of ontogeny, and even ultimately sink to invisibility. Determinants lie far below the limits of visibility, and if there were really only a single determinant to control each cell there would be no chromatin visible in such a case. This objection has in point of fact been urged against me, although I expressly emphasized in advance the assumption that the determinants are continually multiplying throughout the whole ontogeny, so that in proportion as the number of the kinds of determinants lying within a cell diminishes the number of resting determinants of each kind increases. When, finally, only one kind of determinant is present there is a whole army of determinants of that kind.

It follows from this conception of the gradual segregation of the components of the id in the course of development that we must attribute to the determinants two different states, at least in regard to their effect upon the cell in which they lie: an active state, in which they control the cell, and a passive state, in which they exert no influence upon the cell, although they multiply. From the egg onwards, therefore, a mass of determinants is handed on by the cell-divisions of embryogenesis, which will only later become active.

My conception of the manner in which the determinants become active is similar to that suggested by De Vries in regard to his 'Pangens,' very minute vital particles which play a determining part in his 'pangen theory,' similar to that filled by the determinants in my germ-plasm theory. It seems to me that the determinants must ultimately break up into the smallest vital elements of which they are composed, the biophors, and that these migrate through the nuclear membrane into the cell-substance. But there a struggle for food and space must take place between the protoplasmic elements already present and the newcomers, and this gives rise to a more or less marked modification of the cell-structure.