REGENERATION IN EGG AND EMBRYO 221 



It is most difficult to account for these post-generative changes, 

 since the new part has, according to Roux, a double and even a three- 

 fold origin. The pieces of the old nucleus, he admits, may take a part 

 in the formation of the new cells ; wandering cells migrate from the 

 yolk mass of the old half into the new, and the cells of the formed 

 germ-layers may be pushed over to the other side. Since a certain 

 share, and perhaps a large share, of the new cells comes from the 

 hemi-embryo, it is clear that, in addition to the power of self-differen- 

 tiation shown by the uninjured blastomere, we must also ascribe to it 

 certain regenerative powers, at least to the extent that each kind of 

 cell that comes from it can give rise in the injured half to cells like 

 itself, and produce similar structures in the other half. 



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 pro- 

 gressive 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 reexamined 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. 



