486 H. H. NEWMAN 



GENERAL SUMMARY OF CONCLUSIONS 



1. The nature of the influence of the spermatozooii on cleavage and 



development 



The new evidence here offered tends to support the position 

 that the rate of cleavage is largely a function of egg size and egg 

 constitution and is inherited only in this restricted sense, just 

 as certain factors of the environment may be said to be inherited, 

 because the organism finds them ready made for its use. Foreign 

 sperm, when introduced into the egg, may accelerate or retard 

 cleavage or later development, but in so doing is not exercising 

 an hereditary function, for there are cases in which the sperm 

 of a more rapidly developing species retards development and 

 others in which the sperm of a more slowly developing, but closely 

 related species, hastens development. This is surely not hered- 

 ity. I would like to suggest that the influence exerted by foreign 

 sperm upon the rate of development is chemical and mechanical 

 in nature and that it is only when the sperm components of the 

 zygote begin to cooperate with the egg components in bringing 

 about the differentiation of specific characters, that the sperm 

 begins to play its role as a factor in heredity. It is doubtless 

 during the process of gastrulation that the first steps in differ- 

 entiation take place and it is very interesting to note that in 

 so many heterogenic crosses the d*evelopmental stoppages occur 

 at the onset of or during the process of gastrulation. The con- 

 clusion would seem to be obvious then that any teleost sperma- 

 tozoon may play a role in cleavage equivalent to that of agents 

 that are successful in artificial parthenogenesis, but that only 

 certain special kinds of sperm, that can successfully cooperate 

 with the egg nucleus in its hereditary activities, are capable of 

 working out a complete ontogeny. 



