134 DEVELOPMENT OF THE FROG'S EGG [Cn. XII 



I think, to conclude that the power of differentiation lies within 

 the egg itself, and does not depend directly on external stimuli. 

 We have seen that Roux and Weismann (particularly the latter) 

 explain this differentiation of the cells as a result of the quali- 

 tative division of the nucleus from the very beginning of the 

 cleavage. The nucleus, unravelling its qualities at each divi- 

 sion, sends into each cell the proper constituents, and the nuclei, 

 then acting on the cell-protoplasm, cause it to differentiate. 

 On the other hand, Hertwig contends that the early blastomeres 

 are equivalent, and that differentiation is brought about by the 

 interaction of the blastomeres. In other words, any blastomere 

 that has come to occupy a given position has its fate sealed, be- 

 cause in this position it bears a certain relation to the other 

 blastomeres of the whole ; the whole being simply the sum-total 

 of the blastomeres present. But it is impossible to imagine that 

 the interaction of strictly equivalent blastomeres could bring 

 about a self -differentiation. If it is assumed that the gross- 

 contents (such substances as yolk, etc.) determine the differ- 

 entiation of each part, still the hypothesis is obviously insuffi- 

 cient for all cases, because, as we have seen, fragments of any 

 part of the egg of echinoderms develop into whole embryos, 

 and fragments even of the blastula form new blastulse, gas- 

 trulae, and embryos. Some of these small blastulse represent 

 only the " animal " half of the original blastula, and the cells 

 will not, therefore, contain any of the protoplasm or yolk that 

 the cells usually contain that are invaginated, for all this por- 

 tion of the blastula has been cut off. And since these "ani- 

 mal" pieces gastrulate, we must infer that the gross-contents of 

 the blastomere, or collection of blastomeres, do not necessarily 

 cause the differentiation. If, then, neither qualitative division 

 of the nucleus, nor cellular interaction, nor the gross-contents 

 of the blastomeres can be the cause of differentiation of the 

 embryo, what does bring about the differentiation ? There are 

 certain facts of inheritance that also have a bearing on this ques- 

 tion. The characters of the male are known to be transmitted 

 by means of the spermatozoon. The latter carries into the egg 

 mainly the male nucleus. Therefore, many embryologists have 

 turned to the nucleus as the originator of the differentiation of 

 the cell. Various suggestions have been offered as to the way 



