OF THE VERTEBRATE OVARY. 575 



The fibrous layer bounding the membrana propria folliculi is 

 thicker than in the last stage, and the epithelial-like layer (/<?') 

 which bounds it externally is more conspicuous than before. 

 Immediately adjoining it are vascular and lymph sinuses. The 

 thickness of the follicular epithelium at this stage may reach as 

 much as 0*04 mm., though I have found it sometimes consider- 

 ably flatter. The cells composing it are, however, so delicate 

 that it is not easy to feel certain that the peculiarities of any 

 individual ovum are not due to handling. The absence of the 

 peculiar columnar epithelium on the part of the surface adjoin- 

 ing the germinal vesicle is as marked a feature as in the earlier 

 stage. When the egg is nearly ripe, and the vitelline membrane 

 has been reduced to a mere remnant, the follicular epithelium is 

 still very columnar (PI. 25, fig. 23). The thickness is greater 

 than in the last stage, being now about 0*045 mm., but the cells 

 appear only to form a single definite layer. From the character 

 of their nuclei, I feel inclined to regard them as belonging to 

 the category of the smaller cells of the previous stage, and feel 

 confirmed in this view by finding certain bodies in the epithelium, 

 which have the appearance of degenerating cells with granular 

 nuclei, which I take to be the flask-shaped cells which were 

 present in the earlier stage. 



I have not investigated the character of the follicular epithe- 

 lium in the perfectly ripe ovum ready to become detached from 

 the ovary. Nor can I state for the last-described stage anything 

 about the character of the follicular epithelium in the neighbour- 

 hood of the germinal vesicle. 



As to the relation of the follicular epithelium to the vitelline 

 membrane, and the possible processes of its cells continued into 

 the yolk, I can say very little. I find in specimens teased out 

 after treatment with osmic acid, that the cells of the follicular 

 epithelium are occasionally provided with short processes, which 

 might possibly have perforated the vitelline membrane, but have 

 met with nothing so clear as the teased out specimens figured 

 by Eimer. Nothing resembling the cells within the vitelline 

 membrane, as described by His 1 in Osseous Fish, and Lindgren 

 in Mammalia, has been met with 2 . 



1 Das Ei bei Knochcnfischcn. 

 3 Airk.f. Anal. Pliys. 1877. 



