The Evolution Theory, Vol. 1 of 2

by August Weismann

As yet, however, only one group of animals is known to behave demonstrably in this manner, the Diptera among insects; in all others the cell from which the germ-cells exclusively arise, the 'primordial germ-cell,' makes its appearance later in development, usually during embryogenesis and often very early in it, after the first few divisions of the ovum, but sometimes not till long after the end of embryogenesis, and not even in the individual which arises from the ovum, but in descendants which arise from it by budding. This last case occurs especially in the colonial hydroid polyps, which multiply by budding. Here the primordial germ-cell is separated from the ovum by a long series of cell-generations, and the sole possibility of explaining the presence of germ-plasm in this primordial cell is to be found in the assumption that in the divisions of the ovum the whole of the germ-plasm originally contained in it was not broken up into determinant groups, but that a part, perhaps the greater part, was handed on in a latent state from cell to cell, till sooner or later it reached a cell which it stamped as the primordial germ-cell. Theoretically it makes no difference whether these 'germ-tracks,' that is, the cell-generations which lead from the ovum to the primordial germ-cell, are short or very long, whether they consist of three or six or sixteen cells, or of hundreds and thousands of cells. That all the cells of the germ-track do not take on the character of germ-cells must, in accordance with our conception of the 'maturing' of determinants, be referred to the internal conditions of the cells and of the germ-plasm, perhaps in part also to an associated quantum of somatic idioplasm which is only overpowered in the course of the cell-divisions.

This splitting up of the substance of the ovum into a somatic half, which directs the development of the individual, and a propagative half, which reaches the germ-cells and there remains inactive, and later gives rise to the succeeding generation, constitutes the theory of the continuity of the germ-plasm, which I first stated in a work which appeared in the year 1885. Its fundamental idea had already been expressed much earlier by Francis Galton (1872), without however being fully appreciated at the time or having any influence on the course of science, and the same is true with the later theoretical views of Jäger, Rauber, and Nussbaum, all of whom reached the same idea quite independently of each other, and sought to elaborate it more or less fully.

The hypothesis does not depend for support merely on a recognition of its theoretical necessity; on the contrary, there is a whole series of facts which may be adduced as strongly in its favour.

Thus, even the familiar fact that the excision of the reproductive organs in all animals produces sterility proves that no other cells of the body are able to give rise to germ-cells; germ-plasm cannot be produced de novo. An unmistakable corroboration of this, it seems to me, is to be found in the conditions of germ-cell formation in the medusoids and hydroid polyps, for here it is apparent that the birthplace of the germs, that is, the place at which the germ-cells of the animal are formed, has been shifted backwards in the course of phylogenetic evolution, that is, has been moved nearer to the starting-point of development. This shifting has exactly followed the 'germ-tracks.' as we shall see, although in some cases it would have been more advantageous if the birthplace of the germ-cells could have lain outside of these. Obviously, then, it is only the existing cell-generations of the germ-track which were able to give rise to germ-cells, or, in other words, they alone contained the indispensable germ-plasm. With the help of Figs. 94 and 95 I hope to be able to make this matter clear.

Fig. 94. Diagram to illustrate the phylogenetic shifting back of the origins of the germ-cells in medusoids and hydroids. A composite picture. A, branch of a polyp colony. P, polyp-head with mouth (m) and tentacles. St, stalk of the polyp. M, medusoid-bud with the bell (Gl). T, marginal tentacle. m, mouth. Mst, manubrium. GphK, a gonophore-bud. GH, gastric cavity. ekt, ectoderm. ent, endoderm. st, supporting lamella. The germ-cells (kz) arise in the medusoid in the ectoderm of the manubrium—first phyletic stage—where they also attain maturity. In the gonophore-bud (GphK) they arise in the ectoderm (kz´), or further down in the stalk of the polyp at kz´´—third phyletic stage, or in the ectoderm of the branch from which the polyp has arisen, at kz´´´—fourth phyletic stage of the shunting of the originative area of the germ-cells. In the two last cases the germ-cells migrate until they reach their primitive place of origination in the medusoid, or in the corresponding layer of the medusoid gonophore, as may be more clearly seen in Fig. 95. Drawn from my sketch by Dr. Petrunkewitsch.

In the hydroid polyps and their medusoids the germ-cells always arise in the ectoderm; in species which produce sexual medusoids by budding, the germ-cells arise in the ectoderm of the manubrium of these medusoids (Fig. 94, M, kz). But in many species these sexual stages have degenerated in the course of phylogeny into so-called gonophores, that is, to medusoids which still exhibit more or less complete bells, but neither mouth (m) nor marginal tentacles (T), and which no longer break away from the colony to swim freely about, to feed independently, and to produce and ripen germ-cells. The degeneration of the 'gonophores' often goes even further; in many the medusoid bell is represented only by a thin layer of cells, and in some even this token of descent from medusoid ancestry is absent, and they are mere single-layered closed brood-sacs ([Fig. 95], Gph).

The adherence of the sexual animal to the hydroid colony has, however, made a more rapid ripening of the germ-cells possible, and nature has taken advantage of this possibility in all the cases known to me, for the germ-cells no longer arise in the manubrium of the mature degenerate medusoid, that is, of the gonophore, but earlier, before the bud which becomes a gonophore possesses a manubrium. The birthplace of the germ-cells is thus shifted back from the manubrium of the medusoid to the young gonophore-bud (Fig. 94, M, kz). The same thing occurs in species in which the medusoids are liberated, but live only for a short time, for instance, in the genus Podocoryne. Although perfect medusoids are formed, these have their germ-cells fully developed at the time of their liberation from the hydroid colony. But in species in which the medusoid-buds have really degenerated and are no longer liberated, the birthplace of the germ-cells is shifted even further back, and in the first place into the stalk (St, kz´´) of the polyp from the gonophore-buds. This is the case in the genus Hydractinia. In the further course of the process the birthplace of the germ-cells has shifted as far back as to the branch from which the polyp has grown out ([Fig. 94], A, kz´´´); and finally, in the cases in which the medusoid has degenerated to a mere brood-sac (Fig. 95, Gph), even to the generation of polyps immediately before, that is, into the polyp-stem from which the branch arises that bears the polyps producing the gonophore-bud (Fig. 95, kz´´´). Then we find the birthplace of the germ-cells still further back (Fig. 95, kz´´´´), for the egg and sperm-cells arise in the stem of the principal polyps (the main stem of the colony). The advantage of this arrangement is easily seen, for the principal polyp is present earlier than those of the secondary branches, and these again earlier than the polyp which bears the sexual buds, and this, finally, earlier than the sexual bud which it bears. Thus this shunting backwards of the birthplace of the germ-cells means an earlier origin of the primordium (Anlage) of the germ-cells, and consequently an earlier maturing of these.

Fig. 95. Diagram to illustrate the migration of the germ-cells in hydro-medusæ from their remotely shunted place of origin to their primitive place of origin in the gonophore, in which they attain to maturity. The state of affairs in Eudendrium is taken as the basis of the diagram. HP, one of the principal polyps. mu, mouth. ma, gut-cavity. t, tentacle. Sta, its stem. A, a branch of the polyp colony. SP, lateral polyp. Gph, a medusoid-bud completely degenerated into a mere gonophore. Ei, ovum. GH, gastric cavity. st, supporting lamella. The originative area of the germ-cells lies in the stem of the principal polyp at kz´´´´, whence the germ-cells first migrate into the endoderm of the branch (A) at kz´´´, creeping within which they reach kz´´ in the lateral polyp (blastostyle), finally reaching the gonophore (kz) and passing again into the ectoderm. Drawn from my sketch by Dr. Petrunkewitsch.