CH. XI] EFFECT OF IXJURIXG A BLASTOMERE 109 



blastomeres had been killed, we should have anticipated, Roux 

 says, that "hemiembryonfis anteriores" or "posteriores" would 

 have appeared. Roux claims that such forms do really appear. 

 The same result can be obtained, if, after the second cleavage 

 of the egg, two of the four cells be killed, i.e. those two that lie 

 on the same side of the second cleavage-plane. A hemiembryo 

 anterior (?) is shown in Fig. 34, B. It has the anterior end 

 of the medullary folds normally formed, also a normal chorda, 

 mesoderm. and archenteron in this anterior end. In every re- 

 spect it corresponds to the anterior end of a normal embryo, 

 except that the archenteric cavity is small, resulting, Roux 

 thinks, from the impossibility of pushing the yolk-mass poste- 

 riorly, as is done in the normal embryo when the archenteron 

 enlarges. Roux is uncertain whether he has seen any " henri- 

 einbryones posteriores," although one embryo that he found, 

 with thick and short blastoporic lips, may represent such a 

 form. 1 Roux made some further experiments in which one of 

 the first four blastomeres was killed, and other experiments in 

 which three of the first four blastomeres were killed. In the 

 first case he obtained three-fourth morul* and three-fourth 

 blastula?; in the latter case, one-fourth blastulre and one-fourth 

 embryos. Roux concluded from his experiments, "that the 

 development of the frog's gastrula and of the embryo immedi- 

 ately following the gastrula-stage is, after the second cleavage- 

 period, a mosaic work of at least four vertical self-developing 

 (or differentiating) parts." " How far this mosaic work is 

 changed by a change in the position of material in the later 

 development, cannot be determined." 



In later stages in the development of the hemiembryos a new 

 series of phenomena appear, that result in the "reorganization" 



1 We should expect, following Roux's argument, to get as many hemiem- 

 bryones posteriores as anteriores. yet such does not seem to be the case. 

 Hertwig ('93, A) has maintained that it is absurd to suppose the posterior 

 end of the blastopore could appear when there is no anterior end; but this 

 suppositiou rests, I think, on an erroneous idea of the way in which the 

 blastopore forms, for I have shown in my experiments ('94) that the poste- 

 rior lips of the blastopore may appear when the anterior lip has been de- 

 stroyed. The experiment should be carefully repeated with the four-cell 

 stage, where it is possible to distinguish the two anterior and the two posterior 

 cells. 



