THE PHYSIOLOGY OF FERTILIZATION 143 



Finally all eggs completely lose fertilization capacity 

 after fertilization, a problem that we shall discuss in 

 chapter vii. 



We have thus seen reason to believe that the ferti- 

 lizable condition of gametes is associated with the pres- 

 ence of a definite substance in each. The evidence is 

 perhaps more conclusive in the case of the ovum than 

 in the case of the spermatozoon. But we are justified 

 in using this conception as a working hypothesis, and 

 it will be found very useful in subsequent analysis. 



The capacity for parthenogenesis also exhibits par- 

 allel relations to fertilization capacity. Delage (1901 a) 

 was the first to notice this; in his experiments on cyto- 

 plasmic maturation in the egg of the starfish he notes 

 that capacity for fertilization arises first with the break- 

 ing down of the germinal vesicle; this condition lasts 

 until the appearance of the first polar body, but when the 

 second polar body is about to appear fragments are less 

 fertilizable, "even if they do not become completely resist- 

 ant to merogonic fertilization." He finds a 'similar criti- 

 cal period for parthenogenesis; shortly after the germinal 

 vesicle breaks down, parthenogenic agents are peculiarly 

 efficacious; this gradually disappears before the second 

 polar body is formed. "There is a single point in the 

 physiological curve of the egg when the least disturbing 

 action may cause it to turn toward parthenogenesis." 



R. S. Lillie (1915) has made a more detailed exam- 

 ination of this point with reference to heat partheno- 

 genesis in the eggs of the starfish. A short exposure of 

 the eggs to temperatures between 32 and 38 C. will 

 cause the development of almost every egg to a free- 

 swimming larval stage; for each temperature there is 



