24 PROBLEMS OF FERTILIZATION 



favoring fertilization is the action of alkalis in enabling 

 interclass hybridization, discovered by Jacques Loeb. 

 Such experiments furnish important data for the anal- 

 ysis of the reaction, but it is obvious that their inter- 

 pretation must depend upon internal conditions of the 

 fertilization reaction. 



In the third place, the membranes of the egg and of 

 the spermatozoon must influence the occurrence, rate, 

 and extent of the fertilization reaction according to 

 the degree of their permeability to the substances 

 concerned; the egg membrane is of course more espe- 

 cially concerned; its role in the occurrence of partheno- 

 genesis has been studied especially by R. S. Lillie; 

 and I have found in the case of the starfish egg that a 

 resistant egg membrane may entirely block the ferti- 

 lization reaction, though the- block may be removed by 

 agents that render the membrane more permeable. 



The internal conditions of the fertilization reaction 

 may be grouped under three heads: (i) maturity of 

 the germ cells; (2) irreversibility of the reaction; 

 (3) specificity of the reaction. 



i. Maturity. Concerning conditions of maturity of 

 the spermatozoon, but little definitely is known, except 

 that it will not fertilize before its differentiation is 

 complete. Whether the cause of this lies entirely 

 in deficient motility, or partly also in incomplete 

 chemical differentiation, we do not know, though there 

 are some reasons for thinking that the latter factor may 

 be involved. In the case of the ovum our knowledge 

 is in a much more advanced stage. We know that the 

 fertilizable condition, which represents the final maturity 

 of the ovum, arises rather suddenly, usually lasts but 



