164 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 types, 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. 2, 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) and is 
