238 whitman: ' [Vol. I. 



Up to a certain time we see the germinal vesicle held in place 

 by centripetal attraction; then, owing to unknown changes 

 either in itself or in the ooplasm, or in both, repellent action 

 sets in, and it begins to move centrifugally. Now, if it can 

 be shown that the conditions of centripetal attraction remain 

 unchanged, so far as the ooplasm is concerned, the changes 

 which induce repellent action must evidently be located in the 

 germinal vesicle. That no changes take place in the ooplasm 

 which can interrupt the action of centripetal attraction is shown 

 by the deportment of the male pronucleus at the very time 

 when the expulsion of polar globules is in progress, and by the 

 fact that this attraction acts alike ^ on both pronuclei, irrespec- 

 tive of the time, place, or order of their origin. All the essential 

 conditions of centripetal attraction in the ooplasm may then 

 be said to be unaffected by the processes of maturation and 

 fecundation. The primary cause of centrifugal movement must, 

 therefore, lie in the germinal vesicle itself. To ascertain the 

 nature of the causal changes is a task of the future. 



Various causes have been assigned for the centrifugal move- 

 ment of the germinal vesicle. In the meroblastic eggs of 

 vertebrates this movement is connected with the formation of 

 the latebra (Purkinje) ; and in the holoblastic eggs of the Am- 

 phibia, with the origin of the ''figure clavifonne'' (Bambeke). 

 The fact that this movement in the hen's egg follows so closely 

 upon the appearance of the white yolk spherules, might raise 

 suspicion of a passive displacement, the germinal vesicle being 

 pushed upward by the formation of the yolk elements beneath 

 it. This view, advanced by Van Beneden (12; p. 206), is com- 



' It cannot perhaps be said that in all cases this force acts with equal intensity 

 on both pronuclei. In Toxopneustes variegatus, for instance, Selenka (" Befrucht- 

 ung des Eies, etc.," Leipzig, 1878) states that the " Eikern " (female pronucleus) 

 always takes an eccentric position (p. 4), and there remains until after the formation 

 of the male pronucleus. The male pronucleus, on the contrary, makes no delay in 

 its centripetal movement, and, having gained the centre, awaits there the approach of 

 the female pronucleus, which does not appear to move until reached by the astral 

 rays of the former (pp. 7-S). It would appear from Selenka's description that the 

 Eikern is drawn from its eccentric position by nuclear attraction alone. In excep- 

 tional cases the spermatozoon penetrates the egg in the immediate vicinity of the 

 Eikern, and then the two pronuclei unite and move slowly towards the centre (p. 8). 

 Selenka makes no mention of a centrifugal movement on the part of the male pro- 

 nucleus. 



12. Ed. Van Beneden. Recherches sur la composition et la signification de 

 Toeuf. Mem. cour. d. VAcad. roy. des Sciences de Bag., XXXIV., 1870. 



