No. 2.] OOKINESIS. 251 



fectly certain that these eggs have a polar area, comparable 

 with the disc of impregnation in Petromyzon. 



In many teleostean eggs a well-defined germinal disc is pres- 

 ent before impregnation takes place, and in such cases the 

 micropyle is usually described as occupying the centre of the 

 disc. Such a position would indicate, as van Beneden (9, p. 

 376) has pointed out, that the spermatozoon penetrates at a 

 predetermined point. It is probable, however, that the micro- 

 pyle is a little eccentric, and that its polar distance is variable 

 within certain narrow limits, — within an area which may be 

 called the pole of impregnation. 



Prof. Kupffer (25) has discovered in the egg of the trout 

 (thirty m. or more after fertilization) small discs distributed over 

 the surface of the blastodisc, which he compares with the 

 " disque polaire " described by Van Beneden in the &^'g of 

 Ascaris. 



In surface views and under a low magnifying power, these 

 " Polscheiben," a dozen or more in number, appear as light 

 flecks. " Diese Scheiben bestehen aus einer senkrecht gestrich- 

 elten hcllcn Substana, die als Fortsetzimg der dilnnen Dotter- 

 JiaiU erscheint, gegen die Mitte zn an Dicke zimimmt tind hier 

 von eincni Pfropfe dcr Keimsid^stan:^ dnrchbohrt tvird, iiber 

 dessen Oberfliiche sich die Dotterhaut nicht fortzusetzen 

 scheint. An der Basis des Propfes sieht man jederseits zwei 

 kleine Hohlriiume, die Durchschnitte eines die Basis ringsum 

 umziehenden Kanals " (p. 6). 



Kupffer holds that the germinal protoplasm (" Pfropfe ") 

 filling the central perforation of one of these "Polscheiben" 

 fulfils the function of the ^^ bouchon d' impregnation'' of the 

 Ascaris egg; but he has failed to verify this by observation. 



The occurrence of several polar discs is taken as an evidence 

 that more than one spermatozoon is required to fertilize the egg. 

 But such evidence can have very little weight so long as the 

 polar discs themselves remain entirely problematical. Kupffer 

 defends his interpretation on the ground that the discs are simi- 

 lar in structure to the polar disc of Ascaris. But Zacharias 

 has given very strong reasons for believing that the " disque 

 polaire " and its central " bouchon d'impregnation " are artificial 



25. C. Kupffer. Die Befruchtung des Forellenies. Bayerische Fischerei-Zeitiing, 



