24 



THE INITIATION OF DEVELOPMENT: 



of the nucleoplasm of the sperm nucleus. These were able to 

 divide repeatedly, thereby giving rise to new ampiasters (Fig .9). 

 Dalcq concluded from these experiments that the power of di- 

 vision, especially that of forming a bipolar mitosis, is an inherent 

 property of the sperm nucleus, in particular of its nucleoplasm. 

 Yet there is an objection to this conclusion. There are several 



v^N. ^ 



ii//^. 



r-i 



'h. 





^ 



\ 



Pig. 8. Egg of a sea urchin, fertilised with sperm of a mussel, 



Mytilus. (a) fusion of the pronuclei; (b-c) first cleavage, the male 



chromatin (cT) is eliminated; (d) four-cell stage, male chromatin 



in one of the blastomeres. After Kupelwieser. 



exceptions to the rule that co-operation of the male pronucleus 

 is necessary for the formation of a normal bipolar mitosis. In 

 Crepidula (Conklin, 1904) and Ciona (Duesberg, 1926), the 

 female pronucleus will form an amphiaster, even if fusion of 

 the pronuclei is prevented. On the other hand, in Fankhauser's 

 experiments already described (see above p. 17), the super- 

 fluous sperms in the half without the egg nucleus often form 

 a monaster, although, naturally, the nucleoplasm of the sperm 

 is present here. Further, Dalcq's hypothesis does not explain 



