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



blastomeres. 1 On the other hand, the isolated blastomere of the 

 ctenophor-egg develops into a half-embryo. These experiments 

 show that the half-development of the frog's egg need not be 

 the result of the presence of the other blastomere, as has been 

 suggested. This is also shown by Schultze's experiment, in 

 which, although both halves are present and in contact, each 

 blastomere develops into a whole embryo. 



The results show that in general the first blastomeres are 

 totipotent, i.e. each has the power to produce a whole embryo 

 if separated from its fellow, even although it may under cer- 

 tain conditions produce only a half-embryo, as in the frog. 

 Nevertheless, in most forms each isolated blastomere continues 

 to segment as though still in contact with the other half. This 

 latter phenomenon shows that the egg-protoplasm has a defi- 

 nite arrangement according to which the cleavage peculiar 

 to each kind of egg is brought about, and there is sufficient evi- 

 dence to show, I think, that this is a cytoplasmic phenomenon, 

 and is not the result of nuclear interference. We have also 

 seen that some of the isolated blastomeres that cleave as a 

 part may later develop into whole larvie (echinoderms), while 

 other blastomeres that cleave as a part may give rise to half- 

 larvse (ctenophors). That these phenomena too are dependent 

 directly on the cytoplasm is shown by the experiment of cut- 

 ting a piece from the unsegmented egg. Under these circum- 

 stances, the nucleated fragment of the echinoderm-egg gives 

 rise to a whole embryo, although it segments as a part, while in 

 the ctenophor an imperfect embryo is generally formed. The 

 results in these two cases are nearly the same as when the blas- 

 tomeres of the respective eggs are isolated, although in the 

 latter experiment the entire segmentation-nucleus is present. 

 In the ctenophor the process of self-regulation seems to be 



1 Roux ('95) has stated that the development of a half or a whole embryo 

 may depend upon the method employed to separate the blastomeres. If shaken 

 apart, whole embryos result; if cut apart, half-embryos. Zoja's results ('95) 

 refute such an interpretation. He cut apart echinoderm and hydroid eggs and 

 yet got whole embryos. On the other hand, when the blastomeres of the cteno- 

 phor are cut apart, half-embryos result. It must, however, be admitted that 

 disturbance of the contents of an isolated blastomere might be favorable to 

 whole development, as in the frog. 



