THE LIFE-CYCLE OF THE HIGHER ANIMALS AND
ALTERNATION OF GENERATIONS
MODERN embryological doctrine is at least consistent in holding to the three tenets of somatic origin of germ-cells, direct development and epigenesis. If the germ-cells be somatic in origin, direct development is a priori possible. If the hen produce from her body new eggs, it is at all events feasible that any one such egg should develop directly into a new hen; and if this happen, there would appear to be little or nothing to hinder the development from taking on an epigenetic character.*
As, of course, every tyro in embryology knows, the doctrine of epigenesis was established in 1759 by Caspar Freidrich Wolff, who “demonstrated “** in the instance
* Weismann has already more than once (in the preface to the “ Germplasm “ and in the “ Vorträge,” vol. i., p. 385 et seq.) declared against the possibility of an epigenetic development. I find myself in complete accord with him in this conclusion, and my researches may claim to be the first embryological ones to support his attitude in this important matter. My reasons are briefly given in the present section. But, on the other hand, Weismann has never questioned the possibility of either direct development or somatic origin of germ-cells, “in some cases at any rate,” for the higher animals. The writer finds it difficult to understand how he can still accept these two doctrines, which would appear to render an epigenetic development not merely possible, but likely. If all the products of the egg-cleavage be merely bricks for the embryonic body, how are they to form this without the builder, epigensis ?
** It perhaps need hardly be stated that to-day the demonstration of 1759 would require to take on a very different character !