If then, as Roux supposes, the development of the egg consists in an orderly, qualitative series of changes that lead to the subsequent differentiation, we must also suppose that the new parts are gifted with latent powers by virtue of which they can re-create all parts of the other half. Roux supposes, in fact, that each cell carries with it a sort of reserve-plasm, that is dormant in ordinary development but is awakened when any disturbance of the normal development takes place. Objections have been made to this subsidiary hypothesis, since the addition of this to the original assumption of a series of qualitative changes involves such complications that the view can hardly be considered a probable one. This objection is, I think, not as strong as certain critics believe, since the facts of development show beyond a doubt that although the egg has the power of progressive change it has also, as certain experiments show, the power of reorganization, if the ordinary course of events is interrupted. This admission by no means throws us back upon Roux’s hypothesis, for, as will be shown later, a different conception of the development may better account for both phenomena. Inasmuch as a good deal of discussion has taken place in regard to the process of post-generation described by Roux, it should be stated that Endres and Walter reëxamined the process, and found, as had Roux, that the reorganizing cells migrate from the uninjured to the injured side, and around them the protoplasm of that side makes new cells. They found that the injured half is directly overgrown by the ectoderm from the developing half. When the material of the injured blastomere is only incompletely reorganized, there is formed, after post-generation, an embryo that has a protrusion of yolk in the dorsal part of the body. When the injured material is completely worked over, a perfectly formed embryo may result. The typical half-embryos that Roux obtained were also obtained by Endres and Walter. They deny that whole embryos develop from one of the first two blastomeres, as Hertwig affirms. [Illustration: FIG. 62.--After O. Hertwig. _A._ Section through a frog’s egg (blastula stage) in which one blastomere had been killed. _B._ Same. Gastrula stage. _C._ Later gastrula stage. _D, E._ Surface view of embryos from one of first two blastomeres. _F._ Same as last (_E_). Dorsal view. _G._ Ventral view of last. _H._ Dorsal view of another embryo, lying in a very eccentric position. _I._ Later stage of embryo from one blastomere. Other injured blastomere nearly covered over. _J._ Section through gastrula stage of embryo from one of first two blastomeres. _K._ Cross-section of the embryo shown in _F_ and _G_.]