THE PHYSIOLOGY OF FERTILIZATION 163 



In the starfish this condition lasts until after the forma- 

 tion of the first polar globule, but when the second 

 polar body begins to appear the fragments are less 

 fertilizable, even if they do not become completely resist- 

 ant to merogonic fertilization. Wilson made observa- 

 tions similar in many respects in the case of Cerebratulus, 

 and demonstrated in addition that fragments of ferti- 

 lized eggs are completely unfertilizable. Such fragments 

 have a fresh-cut surface and entirely lack any me- 

 chanical protection. 



It will be observed that the fertilizable character 

 of the egg bears no relation to morphological membranes 

 in these cases, for in all we are dealing with freshly cut 

 pieces with an exposed surface. The egg passes from a 

 non-fertilizable to a fertilizable condition, and after 

 fertilization to a non-fertilizable condition again. A 

 fragment of a fertilized egg, devoid of the sperm nu- 

 cleus, does not return to a fertilizable condition. There 

 is no indication that the processes so far are reversible. 



We may express these results by saying that the egg 

 has acquired a physiological protection, as contrasted 

 with the earlier assumed mechanical protection, against 

 polyspermy. 



Superposition of fertilization on parthenogenesis. In- 

 asmuch as fertilization and activation of the egg by 

 parthenogenetic agents produce similar morphological 

 and physiological results on the whole, it would seem 

 that both methods should produce eggs equally resistant 

 to superimposed fertilization. If this were not the case 

 it would seem that artificial activation is not the full 

 equivalent of fertilization with reference to initiation 

 of development in spite of the similarity of results 



