208 WALTER HEAPE. 



situated is continuous undoubtedly with the cortical portion 

 {vide Beneden, loc. cit.). 



When fully mature the vitellus again swells out and there is 

 no space seen between the ovum and the vitelline membrane. 

 At the same time the distinction between cortical and medul- 

 lary portions ceases to be visible, and the female pronucleus 

 probably retires to the centre of the ovum, judging from its 

 behaviour in other typeSj and is no longer to be seen owing to 

 the density of the yolk. In this condition the ovum is fully 

 ripe and is ejected, by the bursting of the follicle, into the 

 funnel-shaped opening of the Fallopian tube. 



Beneden (No. 5) describing the process of the formation of 

 polar bodies in the Rabbit's ovarian ovum, concludes that the 

 germinal vesicle is ejected to form those bodies, and that the 

 ovum becomes therefore a non-nucleated cell, while Balfour 

 (No. 3, vol. i, p. 61) in criticising this statement suggests that 

 further observations " will demonstrate that part of the ger- 

 minal vesicle remains in the ovum to form the female pro- 

 nucleus." 



The latter supposition, I would venture to think, is justified 

 by the observations above recorded, and I would suggest that 

 it is possible the supposed " Monerula " condition of the ovum 

 described by van Beneden was due to the fact that the opacity 

 of the ovum and the retirement of the nucleus to its central 

 portion at the time the observation was made, prevented it 

 from being seen. 



Impregnation. 



Impregnation takes place in the upper portion of the Fallo- 

 pian tube. 



In fig. 10 an ovum is represented which was obtained from 

 the upper end of the oviduct ; it has not yet divided into seg- 

 ments, but spermatozoa have found their way within the zona 

 radiata and two nuclei (the male and female pronuclei) may be 

 seen approaching one another. 



The vitellus is irregularly granular (for the sake of clearness 

 this condition has not been represented in the figure) a^d is 



