200 The Unity of the Organiam 



in the dorso-vcntral plane of the body. He then killed three 

 of the four cells of embryos in the four-celled stage, and in 

 a few instances got something from the remaining cell quite 

 like a quarter embryo. On these observations Roux based 

 the somewhat bold hypothesis that "the development of the 

 frog's gastrula and of the embryo immediately following the 

 gastrula-stage is, after the second cleavage-period, a mosaic 

 work of at least four vertical self-developing (or differen- 

 tiating) parts." To this, however, was added the shelter- 

 ing statement, "how far this mosaic work is changed by a 

 change in position of material in the later development, can- 

 not be determined." 



So striking a series of experiments and so far-reaching 

 an hypothesis were naturally not permitted to stand long 

 without re-examination. O. Hertwig was the first to try the 

 experiment again. His results were quite different from 

 Roux's. He got no hemi-embryos at all, but on the contrary 

 a number of whole embryos, more or less badly deformed in 

 various ways. It should be borne in mind that Roux's 

 method of killing the cells did not remove the injured cells. 

 These remained in full or slightly diminished mass, but 

 wholly inert, as originally supposed, when the operation was 

 entirely successful. Hertwig, on the contrary, believed that 

 usually the life of the pierced cells was not entirely de- 

 stroyed, but that whether quite dead or not, they exerted 

 an important influence on the developing part, and he 

 hazarded the opinion that could one of the two cells be 

 entirely removed, the other would produce a complete em- 

 bryo though of reduced size. 



After much discussion between Roux and Hertwig and 

 others who came into the field, it w^as shown by Morgan that 

 "when the black pole of the uninjured blastomere remained 

 up, the blastomere developed in all cases observed into a 

 half -embryo. Conversely, those eggs in which the white pole 

 was turned upward, formed, in most cases, whole embi'yos 



