240 WHITMAN. ' [Vol. I. 



definite relation with any of the internal changes before con- 

 sidered? If the view presented in the preceding pages be 

 tenable, the period of receptivity may be said to date, not from 

 the expulsion of the polar globules, but from the moment the 

 conditions of centripetal attraction are reversed in the germinal 

 vesicle. A period of non-saturation begins with the centrifugal 

 movement of the germinal vesicle, and terminates with the 

 penetration of the spermatic body. As soon as all the elements 

 of saturation are present, external manifestations of centripetal 

 attraction cease, and there remains only the work of internal 

 equilibration, which ends with the centripetal march of the 

 pronuclei. 



From this standpoint, it is idle to talk about mechanical 

 contrivances for preventing the admission of supernumerary 

 spermatozoa, as if the receptivity of the ovum were not self- 

 regulating. The idea that the micropyle may be closed against 

 spermatozoa by a polar globule, as held by Hoffmann (i6, p. 

 68), is inadmissible, as will be shown by observations soon to 

 be published. Allowing Hoffmann's observation to be correct, 

 what grounds have we for supposing that spermatozoa could 

 not pass directly through a polar globule lodged in the micro- 

 pyle? Has not the penetration of polar globules by sperma- 

 tozoa been repeatedly observed? And is it at all probable that a 

 polar globule would prove more renitent within the micropyle 

 than elsewhere? 



Equally untenable is the suggestion of Calberla (17, p. 458), 

 that the tail of the spermatozoon is left in the micropylar canal 

 for the purpose of blocking the way to other spermatozoa.^ 



In the Echinoderm egg, according to Fol (6, p. 94), it is 

 the rapid formation of an impenetrable vitelline membrane, at 

 the moment when the spermatozoon comes in contact with the 

 vitellus, which makes it impossible for other spermatozoa to 

 enter. There is no evidence that superfetation would follow 

 if such a membrane were not formed;- hence its formation 

 would not necessarily be connected with any such function as 

 Fol has ascribed to it. 



16. C. K. Hoffmann. Zur Ontogenie der Knochenfische. Amsterdam, 1881. 



17. E. Calberla. Der Befruchtungsvorgang beim Ei von Petromyzon Planeri. 

 Zeilschr. f. wiss. Zool., XXX., 1S77. 



' Kupffer and Benecke affirm, positively, that the tail is not left in the micropyle. 



