CH. XII] IXTERPRETATIOXS AXD COXCLUSIOXS 131 



if shaken in a small tube. * Those fragments which contain the 

 egg-nucleus may be fertilized and will develop. If the pieces 

 are large enough a gastrula is formed, and still larger pieces 

 develop into normally formed larvte. 



When the unsegmented egg of the ctenophor is cut into pieces, 

 there may result either a whole larva or a larva lacking certain 

 parts, and, further, the study of the cleavage of these egg- 

 fragments shows that if the fragment cleaves like the whole 

 egg (but with smaller blastomeres) then a whole larva results, 

 while if the cleavage is irregular the larva is also imperfect. 

 Presumably, in the first case the egg has been cut symmetri- 

 cally, but in the second case unsymmetrically. Or we might 

 assume that in the one case the egg-fragment rearranged its 

 protoplasm into a new whole, while in the second case it 

 was unable to do so. On either alternative we must conclude 

 that a defect in the protoplasm often brings about a modified 

 cleavage and also a defective embryo, and this takes place even 

 although the whole of the nuclear material of the unsegmented egg 

 r cumins present. There seems, therefore, no escape from the 

 conclusion that in the protoplasm and not in the nucleus lies the 

 differentiating power of the early stages of development. 



GENERAL COXCLUSIOXS 



We have seen that one of the first two blastomeres of the 

 frog's egg may develop into a half-embryo, or into a whole 

 embryo of half-size, according to the conditions of the experi- 

 ment. So long as the first two blastomeres remain in contact 

 without any disturbance of the cell-contents, each blastomere 

 develops its half of the body. On the other hand, if the proto- 

 plasm is disturbed by reversing the position of the egg after 

 the first cleavage, there generally results a whole embryo from 

 each blastomere. Unfortunately, it has not been found possible 

 to separate completely from each other the first two blasto- 

 meres of the frog's egg, so that we do not know whether 

 a whole embryo of half-size or a half-embryo would result. 

 In other animals (Echinodermata, Hydromedusee, Teleostei, 

 Amphioxus, Ascidia, and Salamandra) each of the first two 

 blastomeres, if separated from its fellow, develops into a wiiole 

 embryo, regardless of the means employed to separate the 



