THE PHYSIOLOGY OF FERTILIZATION 155 



Its loss therefore constitutes a very striking chemical 

 change directly due to f ertilization. A similar loss follows 

 the effective action of parthenogenetic agents which 

 render the eggs insusceptible to fertilization (Lillie, 1914; 

 Moore, 1916). 



IV. PARTIAL FERTILIZATION 



As fertilization is a series of processes it should be 

 possible to arrest the series at various stages and thus 

 to discover the value of the various steps for develop- 

 ment. The experiments which have been directed to- 

 ward this end are not very numerous, as considerable 

 difficulties exist in arresting the progress of fertilization 

 without destroying the entire zygote. 



The writer (1911-12) found ideal material for solu- 

 tion of part of this problem in the egg of Nereis. As 

 we have previously seen, the spermatozoon produces 

 the cortical changes before its penetration into this egg, 

 which does not occur until about forty-five minutes 

 after initiation of the cortical changes. During this 

 time it is imbedded in the jelly secreted by the egg. 

 If the egg be centrifuged at the proper time, the jelly, 

 which is of much less specific gravity than the egg, 

 separates from it completely, and in so doing frequently 

 removes the spermatozoon. That the centrifuging pro- 

 cess itself is not deadly is proved by the fact that those 

 eggs from which the spermatozoon is not removed 

 always segment and may develop approximately nor- 

 mally. 



The eggs from which the spermatozoon is removed 

 complete the maturation process which is initiated by 

 the spermatozoon, but they never segment. The egg 



