MICHIGAN ACADEMY OF SCIENCE. 65 



of embrvologists. The entrance of the sperm into the enucleated egg, 

 8ini}i]y restores the power it had lost, owing to the deprivation of the 

 nnclens. The entrance of the sperm into the egg; which still retains its 

 nucleus, at once brings it to the condition ordinarily and necessarily 

 immediately i>recedent to division ; that is, it doubles the number of 

 chromosomes. It is, then, unnecessary for it to '"rest'' — which is to say, 

 take the time necessary for the stages involved in the dividing of the 

 chromosomes — and it immediately proceeds to inaugurate the develop- 

 mental processes. 



It must be remembered that this process was begun when the full 

 number of chromosomes was present in both bodies. In the division 

 that now takes place, the distribution of these chromosomes plays the 

 important part. It is evident that those in which the chromosomes from 

 each source were evenly distributed would have the advantage over the 

 others and natural selection (or natural elimination) would in time 

 establish these as the dominant type. 



All this, were the conditions suitable for immediate development. But 

 the very thing that led to the evolution of dimorphic individuals was 

 the pressure of hardship which necessitated deferred development. The 

 same causes, then, which were effective in producing the inequality of 

 division that results in the "elimination" of the first polar body, again 

 operate, and this at once, to divide off the second polar body. 



The fertilized egg now contains the normal number of chromosomes 

 and the second polar body the same. From this condition has been 

 evolved the reducing division by a gradual shifting backward in de- 

 velopment of a segregation of the chromosomes in both sperm and egg. 

 In this way perfect fertilization, by which is meant equality in number 

 of chromosomes from each source, has been effectively secured. That 

 this segregation has not progressed to the same extent in sperm and 

 egg is significant. The active spermatozoon has carried the process to 

 the extreme of segregation before the last formative division; the more 

 passive ovum scarcely completes it before fertilization is accomplished. 



Albion, Mich., April, 1910. 

 9 



