INITIATION OF DEVELOPMENT IN EGG OF ARBACIA. 399 



fertilizability. This period is likewise the optimum for experi- 

 mental parthenogenesis. In some cases this fertilizability we 

 know is due to the presence in or at the cortex of a substance, 

 fertilizin. Complete fertilization-reaction depends upon the com- 

 bination of fertilizin and sperm. The cortical explosions leading 

 to membrane separation are the sign and sequela of this complete 

 fertilization-reaction. There is evidence that in experimental 

 parthenogenesis also the same phenomena obtain (Lillie, '14, '20% 

 '20 b ; Moore, '16, '17; Just, '15, 'ic/, '19"). 



Now, in fertilization the primary object is the incorporation of 

 the sperm nucleus to the end that chromosomes of each parent are 

 alike present in the ensuing division. This object is attained by 

 the reaction between sperm and fertilizin by which the sperm head 

 is made to swell and to form an aster out of aster-forming sub- 

 stance present in the egg. The sperm thus carries the aster to 

 the egg nucleus and cell division ensues. 



There is evidence that indicates that the aster-forming substance 

 and fertilizin are not identical, though they may be spatially re- 

 lated. The work of Delage, Wilson Yatsu, and R. S. Lillie shows 

 that in the eggs studied the capacity for merogeny, fertilization, 

 and experimental parthenogenesis depends upon the presence in 

 the cytoplasm of material from the germinal vesicle. We might 

 interpret this to mean that the fertilizability depends upon the 

 presence of fertilizin alone, and that fertilizin and the aster- 

 forming substance are identical. But on this basis how shall we 

 account for fertilization in Nereis and Platyncrcis? In these eggs 

 the fertilization-reaction takes place while the egg is in the germi- 

 nal vesicle stage. At this stage fertilizin is already at the cortex. 

 The sperm aster, on the other hand, never forms until the sperm 

 is in the endoplasm into which germinal vesicle sap has diffused. 

 The sperm aster arises similarly in the eggs of Chcctoptcnis and 

 of Allolobophora. Where, as in the eggs of Arbacia and of 

 Echinarachnins, the fertilization-reaction normally takes place in 

 the mature egg, the germinal vesicle material by diffusion has 

 previously reached the ectoplasm ; the sperm aster forms, there- 

 fore, shortly after the sperm passes the cortex. 



In experimental parthenogenesis as in fertilization cell division 

 depends upon the localization of aster-forming substance around 



