OF THE VERTEBRATE OVARY. 567 



nest, and the 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 shew that the primitive 

 ova in the female are not to be regarded as true ova, but 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, some- 

 what flattened, follicular epithelium (PI. 24, 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. 24, fig. 7, 

 and PI. 25, 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 O'o6 mm., 

 and their nucleus of about 0^04 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 



