No. 2.] OOKINESIS. 239 



pletely disproved by the mode of origin of the latebra ( 1 3 , p. 6^^ . 

 Oellacher's (14, pp. 18, 24) theory of the expulsion of the 

 germinal vesicle by contractions of the vitellus has been disposed 

 of by more recent observations. 



The idea of passive displacement has recently been amplified 

 to a general theory by Ryder (15, pp. 95-101). When we re- 

 flect that this movement is common to all types of eggs, that 

 in the majority of cases it takes place after the tgg has attained 

 its full size, and after the formation of the food-yolk is 

 completed, we find it difficult to accept this mode of explana- 

 tion, and quite impossible to concede that the " law of dis- 

 placement'' x^ to be found in this direction. The holoblastic 

 eggs mentioned by Ryder (p. loi), not to mention "numerous 

 others, furnish evidence quite fatal to his theory. Take the 

 single example of the frog's egg, where the germinal vesicle 

 maintains a nearly central position until a short time before 

 deposit, taking up its centrifugal march after the food-yolk is 

 all present. If the germinal vesicle is simply crowded to the 

 surface by food-yolk, how can the penetration of the yolk by 

 the pronuclei be accounted for? Equally fatal to the theory 

 is the persistence of the latebra and the cord of protoplasm con- 

 necting it with the cicatricula. 



II. Receptivity of the Ovum for Spermatozoa. 



The idea seems to have been widely accepted that the acces- 

 sibility of the ovum to the spermatozoa is regulated by external, 

 mechanical means, rather than by internal, physiological con- 

 ditions. It is well known that the period of fecundation 

 is, generally speaking, relatively a short one ; but the distinc- 

 tion between receptivity and accessibility is very generally 

 ignored. 



If centripetal attraction is persistent, as maintained in the 

 foregoing chapter, why should the ovum enjoy only a transitory 

 receptivity? Can the duration of this period be brought into 



13. WiLH. Waldeyer. Eierstock und Ei. Leipzig, 1870. 



14. J. Oellacher. Beitrage zur Geschichte des Keimblaschens im Wirbel- 

 thiereie. Arch. f. mik. Anat., VIII., 1872. 



15. John A. Ryder. " Embryography of Osseous Fishes." Anmtal Report 

 of the Commissioner of Fish and Fisheries for 1882. 



