158 PROBLEMS OF FERTILIZATION 



second phase is much more gradual in most forms than 

 it is in the sea urchins and usually gives the impression 

 of being a continuous process. 



We have seen previously that the spermatozoon 

 bears a substance essential for fertilization, which is 

 readily lost. We may therefore suppose that it is this 

 substance which activates the cortical changes of the egg. 



The questions may now be raised: (i) How soon 

 after entrance of the spermatozoon into the egg is the 

 activation process completed? (2) Is there a quanti- 

 tative relation in the internal phenomena of activation ? 

 Wilson (1903) observed that if the fertilized eggs of 

 Cerebratulus be cut in two shortly after the penetration 

 of the spermatozoon in such a way that one part con- 

 tains the sperm nucleus and the other the egg nucleus, 

 only the former develops, while the part containing the 

 egg nucleus completes its maturation but goes no farther; 

 it was therefore incompletely fertilized at this time. 

 Ziegler (1898) observed that if the egg of the sea urchin 

 be so constricted after fertilization that one part con- 

 tains the sperm nucleus and the other the egg nucleus, 

 the part that contains the sperm nucleus undergoes 

 cleavage and develops farther; in the other part the 

 egg nucleus undergoes remarkable transformations, dis- 

 solving and reappearing, a process which is repeated 

 several times, but this part does not undergo cleavage. 

 In spite, therefore, of the presence of the sperm nucleus 

 in a constricted portion of the same egg, the part con- 

 taining the egg nucleus was not completely fertilized. 

 Yet we know from experiments on artificial partheno- 

 genesis that the egg nucleus is capable of complete 

 activation without participation of the sperm nucleus. 



