420 



INHERITANCE AND DEVEIOPMENT 



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

 forming part of an entire developing egg. In Amphioxus or Clytia 

 this tendency is successful almost from the beginning. In other 

 forms the hmiting 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 Ggg, Ilyanassa. 

 [Crampton.] 



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

 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 miseg- 

 viented egg, containing the normal egg-nucleus, gives rise to a defec- 

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



