No. 2.] OOKINESIS. 



241 



Besides, Hertwig (18, p. 173) has shown quite conclusively 

 that this membrane is already present before any spermatozoon 

 reaches the vitellus, and concludes that it is not a mechanism 

 for limiting the number of spermatozoa admitted. Hertwig 

 remarks (p. 173), that the existence of membraneless eggs is 

 against the idea advanced by Fol, and adds, — '' It seems to me, 

 therefore, that it is the egg-plasma itself which alone, during 

 unimpaired vitality, can prevent the entrance of more than otie 

 spermatozoon. At all events, this pJienomenon finds its analogue 

 iji the copulation of the lowest unicellular plants and anij^ials, 

 where one also sees only two cells uniting in tJie sexual act." 



Selenka (19, p. 4) places himself on the side of Fol. 



Kupffer and Benecke (20, p. 21) have advanced the idea 

 that the spermatozoon is drawn into the vitellus by an attrac- 

 tive influence emanating from an egg-nucleus. Mutual attrac- 

 tion between the vitellus and this nucleus carries the latter from 

 its place of origin near the formative pole, towards the centre of 

 the egg; and as the attraction between the nucleus and the 

 spermatozoa diminishes as the distance increases, only the fore- 

 most spermatozoon may be supposed to keep within its in- 

 fluence, all the rest being left behind. As this view is plainly 

 incompatible with what is now known about the movements of 

 the pronuclei, it requires no further notice. 



We are not infrequently told that, when the spermatozoon 

 enters the egg before maturation is complete, it remains un- 

 changed and inactive in the periphery of the egg until the polar 

 globules are ejected. It is evident, however, that observations 

 on this point have not been sufficiently close and searching in 

 many cases, as often happens when observations are made at 

 random, and in ignorance of their theoretical bearings. The 

 time of appearance of the male aster, in some cases, would raise 

 a suspicion that centripetal attraction manifests itself more 

 strongly after the elimination of the polar globules than before ; 

 but I have failed to find a single well-ascertained fact in support 



18. O. Hertwig. Beitrage z. Kenntniss d. Bildung, Befruchtung u. Theilung 

 d. thier. Eies. Dritter Theil. I. Abschnit. Morph. Jahrb., IV., 1878. 



19. Emil Selenka. "Befruchtung des Eies von Toxopneustes variegatus." 

 Leipzig, 1878. 



20. Kupffer and Benecke. Der Vorgang der Befruchtung am Eie der Neunau- 

 gen. Kcinigsberg, 1878. 



