420 



INHERITANCE AND DEVEIOPMENT 



tend to confine its operations to the role it would have played if still 

 forming part of an entire developing ^g^. In Amphioxits or Clytia 

 this tendency is successful almost from the beginning. In other 

 forms the Umiting conditions are only overcome at a later period, 

 while in the ctenophore or snail they seem to afford an insurmount- 



Fig. 190. — Partial development of isolated blastomeres of the gasteropod ^gg, Ilyanassa. 

 [Crampton.] 



A. Normal eight-cell stage. B. Normal sixteen-cell stage. C. Half eight-cell stage, from 

 isolated blastomere of the two-cell stage. D. Half twelve-cell stage succeeding. E. Two stages 

 in the cleavage of an isolated blastomere of the four-cell stage ; above a one-fourth eight-cell stage, 

 below a one-fourth sixteen-cell stage. 



able barrier to complete development. What determines the limita- 

 tions of development in these various cases ? They cannot be due to 

 nuclear specification ; for in the ctenophore the fragment of an iniseg- 

 mented ^%^, containing the normal egg-nucleus, gives rise to a defec- 

 tive larva ; and my experiments on Nereis show that even in a highly 



