STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARY. 399 



separation of the nucleus with its protoplasm as a distinct ovum. 

 2. By the direct metamorphosis of an isolated primitive ovum 

 into a permanent ovum. The difference between these two 

 modes of formation does not, from a morphological point of 

 view, appear to be of great importance. 



The above results appear clearly to show that tlie primilive 

 ova hi the female are not to be regarded as true ova, hut as the 

 parent sexual cells which give rise to the ova : a conclusion which 

 completely fits in with the fact that cells exactly similar to the 

 primitive ova in the female give rise to the spermatic cells in 

 the male. 



Slightly after the period of their first formation the permanent 

 ova become invested by a very distinct and well-marked, somewhat 

 flattened, follicular epithelium (PI. XVII^, fig. 3). Where the 

 ova lie in the deeper layers of the germinal epithelium, the follicular 

 epithelium soon becomes far more columnar on the side turned 

 inwards, than on that towards the surface, especially when the 

 inner side is in contact with the stroma (PI. XVII, fig. 7, and 

 PI. XVIII, figs. 24 and 26). This is probably a special provision 

 for the growth and nutrition of the ovum. 



There cannot be the smallest doubt that the follicular epithe- 

 lium is derived from the general cells of the germinal epithelium — 

 a point on which my results fully bear out the conclusions of 

 Ludwig and Semper. 



The larger ova themselves have a diameter of about 006 mm., 

 and their nucleus of about 0"0-i mm. The vitellus is granular, 

 and provided with a distinct, though delicate membrane, which 

 has every appearance of being a product of the ovum itself rather 

 than of the follicular epithelium. The membrane would seem 

 indeed to be formed in some instances even before the ovum 

 has a definite investment of follicle cells. The vitellus is 

 frequently vacuolated, but occasionally the vacuoles appear 

 to be caused by a shrinking due to the hardening reagent. 

 The nucleus has the same peculiar reticulate character as at 

 first. Its large size, as compared with the ovum, is very 

 noticeable. 



With this stage the embryonic development of the ova comes 

 to a close, though the formation of fresh ova continues till com- 

 paratively late in life. I have, however, two series of sections 

 of ovaries preserved in osmic acid, from slightly larger embryos 

 than the one last described, about which it may be well to say a 

 few words before proceeding to the further development of the 

 permanent ova. 



The younger of these ovaries was from a Scy Ilium embryo 10 

 centimetres long, preserved in osmic acid. 



A considerable number of nests were present (PI. XYIII, 



VOL. XVIII. NEW SER. D D 



