2 1 8 HE GENERA TION 



that its development is prevented. The same needle, without heating 

 again, was used for one or two other eggs, for, if the needle had been 

 so hot in the first instance that both blastomeres had been injured by 

 the heat, this might not happen in the second or the third egg. It 

 was found that amongst the eggs that had been operated upon in this 

 way, some had been so much injured that neither blastomere developed, 

 others had been so little injured that both blastomeres developed, but 

 in the successful operations the uninjured blastomere developed, while 

 the injured one did not. In the last case the uninjured blastomere 

 divided, and produced a large number of cells. A segmentation 

 cavity was present in the upper part of the hemisphere (Fig. 61, A). 

 The injured half remained in contact with the other, completing the 

 sphere, but it did not segment. A half-embryo developed from the 

 uninjured half, as shown in Fig. 61, B, C. This embryo has a half- 

 medullary fold along the side in contact with the injured half. At the 

 anterior end somewhat more than half a head is present, and at the 

 posterior end there is a half-blastopore. The cross-sections 1 (Fig. 61, 

 C), through the embryo, show that beneath the half-medullary fold a 

 rod-like notochord is present, which is made up apparently of fewer 

 cells than the normal notochord, but it has, in cross-section, a round 

 and not a half form. At the side, the mesoderm is present, as in the 

 normal embryo, and it has produced the characteristic mesoblastic 

 somites. An archenteron is formed in the half-embryo, and, since it 

 is smaller than the normal, it may, perhaps, be called a half-archen- 

 teron. The embryo is, therefore, in most respects a half-structure. 

 The head is, however, nearly a whole head, but whether this is due 

 to a whole head developing out of material derived entirely from one 

 of the two blastomeres, or whether, as Roux supposes, a portion of 

 the material of the injured blastomere has been worked over, i.e. 

 "post-generated," remains, I think, an open question. 



The results of this experiment seem to confirm Roux's conjecture 

 that the material of each of the first two blastomeres is of such a sort 

 that it gives rise to half the embryo, and, if so, there would be some 

 probability that there is a causal connection between the first cleavage 

 and the separating out of the parts of the embryo. In fact, Roux drew 

 this conclusion, and even attempted to show how such a qualitative 

 division is brought about. It should not be overlooked, however, that 

 this conclusion goes beyond the legitimate bounds of deduction from 

 the results, since the half-development takes place while the injured 

 half retains its connection with the developing half, the former still 

 remaining alive. On the other hand, the presence of the injured half 

 makes the experiment more suitable to demonstrate that each of the 

 first blastomeres gives rise, under normal circumstances, to half of the 



1 The cross-section C is reversed as compared with the half-embryo B. 



