22 PROBLEMS "OF FERTILIZATION 



must be the effective one, but no common factor has 

 been found, or can be found, in the agents themselves; 

 the only common factors are in the reproductive cells. 

 This leaves the method of parthenogenesis in the same 

 position as the method of analysis; that is, in the position 

 of determining what are the changes in the egg itself 

 that initiate development, and what is the nature of their 

 dependency upon the external agent or spermatozoon. 

 The answer to these questions cannot proceed exclusively 

 from parthenogenetic studies, though, to the extent that 

 the same questions are involved, parthenogenesis and 

 f ertilization studies must furnish the same answer. But 

 there are obviously fundamental problems of fertilization 

 that cannot be touched by methods of artificial partheno- 

 genesis. 



The conditions to be fulfilled in fertilization involve, 

 not only penetration of the spermatozoon, or some part 

 of it, into the egg, but also reactioiTbetween the two, 

 which is evidenced by the behavior of both partners; 

 for it is possible for a spermatozoon to penetrate an egg 

 and no reaction to be evidenced in the behavior of either 

 the egg or sperm, as when immature eggs are penetrated 

 by mature spermatozoa. We may therefore speak of 

 SL fertilization reaction when the behavior of both partners 

 indicates that the process is proceeding normally. 

 Fertilization has its quantitative aspect, and the reaction 

 may be complete or exhibit varying degrees of incom- 

 pleteness. For a normal fertilization reaction certain 

 internal conditions of the partners and certain external 

 conditions of the medium must be realized. The study 

 of the external conditions throws light upon the reaction, 

 because the nature of the internal conditions may be 



