The Evolution Theory, Vol. 1 of 2

by August Weismann

As to the fundamental ideas expressed in the theory, I have already shown that these remain unaltered, even if we do not assume a disintegration or segregation of the germ-plasm, but think of all the developing cells as equipped with the complete germ-plasm. In that case the determinants would be liberated to activity solely by specific stimuli. But in regard to the assumption of disintegration, it must be noted that the facts cited relative to the sea-urchin's ova do not by any means hold true of the eggs of all animals.

In various animal types each of the first two segmentation-cells, when separated from its neighbour, produces only a half-embryo, and any one of the first four cells a quarter-embryo. This 'fractional embryo' is, however, in some cases able later to develop into a whole embryo (to 'postgenerate' itself, as W. Roux says). The isolated blastomere shows, to begin with, an activity of only a half of the primary constituents of the animal, as was first established by W. Roux and maintained conclusively, in spite of many attacks, until it was established beyond doubt by the detailed corroboratory investigations of Endres. The secondary completion of the embryo, which, however, is still disputed, must be regarded as a regeneration, and, to explain it, a co-operation of the complete but not yet wholly active germ-plasm in both segmentation-cells must therefore be assumed.

It would carry us too far if I were to deal in detail even with the most important of the numerous facts that the last decade has brought to light; I shall restrict myself to the most essential.

That isolated segmentation-cells have the capacity of developing into embryos which are complete but correspondingly smaller in size has been demonstrated in animals of various groups, though it does not seem to go to the same length in all. In the Medusæ we find that not only one of the first two, but one of the first four, eight, and even sixteen segmentation-cells may develop a whole larva when isolated (Zoja). In the sea-urchin at least any one of the first eight blastomeres may do so. And Driesch's experiments in cutting up the young larvæ at the blastula-stage (a single-layered ball of cells) leads us to assume that each of these cells still possesses the complete germ-plasm. Beyond that stage, however, the primary constituents obviously divide into those of the ectoderm and those of the endoderm, for the subsequent two-layered stage in the sea-urchin's development, the gastrula, does not complete itself if it be artificially divided into fragments which consist only of cells from the outer, or only of cells from the inner layer. In corroboration of this experiment made by Barfurth, Samassa was able to demonstrate in regard to the egg of the frog that, even after the third division of the ovum, the segmentation-cells are so different from each other in respect of their primary constituents that they were not able to replace each other mutually. When this investigator killed the ectoderm-cells alone by means of an induction current, or the endoderm-cells alone, the dead half could not be replaced by the half which remained alive, and the whole ovum perished.

If these facts may be adduced in favour of a separation of the primary constituents at an earlier or later stage, we find even stronger proofs among the Ctenophores, Gastropods, Bivalves, and Annelids. In the last-named group Wilson has shown it to be probable that development is really a 'mosaic work,' as Roux and I had assumed. The older observations made by Chun at an earlier date on the Ctenophora, and the more recent experiments of Fischel on the same animals, prove the same thing for this group. In this case complete larvæ are easily distinguished from mere 'partial developments' by the number of the characteristic 'ciliated meridional rows' or ribs, which extend from one pole of the larva to another. In the complete larva there are eight of these, but in larvæ from one of the first two blastomeres (isolated) there are only four, and in those which have arisen from one of the first four blastomeres there are only two. If an ovum at the eight-cell stage can be successfully divided into separate blastomeres, each of these will form an 'eighth larva,' always with only one ciliated rib. Even in the succeeding sixteen-cell stage it could still be demonstrated that the substance responsible for the formation of the ribs only lies in particular places and always suffices only for eight ribs. The sixteen-cell stage consists of eight large cells and eight small ones, the 'macromeres' and the 'micromeres'; if an ovum at this stage be cut so that one piece contains five macromeres and five micromeres, a partial larva will develop which possesses only five ribs, while the larva from the other portion will have only three. But the localizing of the rib-determinants can be followed still further, for in larvæ in which individual micromeres have been displaced from their normal position there is a correlated displacement of the corresponding ribs, and a dislocation of their ciliated comb-plates. The determinants of the ribs must therefore lie in the micromeres, and we must conclude that at the antecedent division they were only imparted to one daughter-nucleus, while the other, that of the macromere, did not receive this kind of determinant. Here then we have an example of dissimilar or differential division. Those who oppose this theory of qualitative division will hardly be likely to admit this, but will rather seek to maintain that 'external influences,' such as relative position, determine which cells are to give rise to the ciliated ribs and which are not. But the fact that artificial displacement of the micromeres leads to a disarrangement of the ciliated comb-plates, of which the ribs are made up, invalidates this suggestion, and at the same time overthrows the interpretation that it may be the cells which lie on particular meridians that are determined by this position to the production of ciliated plates. Obviously, the converse of this is true; those cells which contain the rib-determinants come to lie in the regular course of development in these eight meridians, and the cells lying between them, though of the same descent (from micromeres), contain no such determinants and therefore form no ribs. But if those cells which are equipped with rib-determinants be artificially displaced, then they give rise to swimming-plates elsewhere than on the aforesaid meridians.

The experiments made by Crampton on a marine Gastropod, Ilyanassa, likewise go to prove that a disintegration or segregation of the primary constituents does occur in the course of development. In this case, when the first two or first four segmentation-cells were artificially separated from each other, they developed exactly as if they still belonged to the complete ovum, that is, each isolated segmentation-cell yielded, respectively, a half or a quarter-embryo. And these 'partial embryos' were not able in this case to give rise subsequently to the missing parts or to form complete embryos.

There are thus two contrasted groups of animals, in one of which a segregation of the mass of primary constituents apparently takes place at the very beginning, while in the other it does not take place in the first stages of development, but apparently occurs later on. We may distinguish these two groups, with Heider, as those having 'regulation ova' and those having 'mosaic ova.' But I do not see that this affords any reason why we should give up our conception of the successive segregation of the germ-plasm into its determinants, even although, as I said before, I may modify it so far as to say that the segregation does not necessarily take place in all groups and species of animals at the same time, but occurs earlier in some and later in others.

Now that I have shown how the germ-plasm theory may be brought into harmony with the phenomena of ontogeny, I wish to go on to show what the theory can accomplish in clarifying our understanding of the phenomena of reproduction and heredity. I shall at the same time give a brief exposition of some of the most important of these phenomena.

First, a few words in regard to the development of the reproductive cells. We may leave aside in the meantime the question whether they are sexually differentiated or not; we are only concerned just now with the main problem: How is it possible for the organism to produce germ-cells, that is, cells which contain the complete germ-plasm with all its determinants, when the building up of the body in ontogeny, according to our theory, involves a disintegration or segregation of the determinant-architecture into smaller and smaller groups? It is impossible that specific determinants should arise de novo, just as an animal cannot arise otherwise than from its germ, nor a cell otherwise than from a cell, nor a nucleus otherwise than from an already existing nucleus. If vital units ever originate de novo at all, it is only conceivable in the case of the very simplest biophors, as we shall see later when we come to speak of 'Spontaneous Generation.' Specific biophors and the determinants composed of them have behind them a phylogeny, a history, which conditions that they shall arise only from their like.

Thus we see that germ-cells can only arise where all the determinants of the relevant species arranged as ids are already present. If we could assume that the ovum, just beginning to develop, divides at its first cleavage into two cells, one of which gives rise to the whole body (soma) and the other only to the germ-cells lying in this body, the matter would be theoretically simple. We should say, the germ-plasm of the ovum first doubles itself by growth, as the nuclear substance does at every nuclear division, and then divides into two similar halves, one of which, lying in the primordial somatic cell, becomes at once active and breaks up into smaller and smaller groups of determinants corresponding to the building up of the body, while the germ-plasm in the other remains in a more or less 'bound' or 'set' condition, and is only active to the extent of gradually stamping as germ-cells the cells which arise from the primordial germ-cell.