406 F. M. BALFOUR. 



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 mem- 

 brane has been reduced to a mere remnant, the follicular 

 epithelium is still very columnar (PI. XYIII, fig. 23). The thick- 

 ness is greater than in the last stage, being now about 0045 mm., 

 but the cells appear only to form a single definite layer. From 

 the character of their nuclei, I feel inclined to rrgard 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 mem- 

 brane, as described by His,i in Osseous Fish, and Lindgren in 

 Mammalia, has been met with.^ 



My observations in Raja are not so full as those upon Scyllium, 

 but they serve to comj)lete and reconcile the observatioTis of 

 Semper and Schultz, and also to show that the general mode of 

 growth of the follicular epithelium is fundamentally the same 

 in my representatives of the two divisions of the Elasmobranchii. 

 In very young egg;$, in conformity with the results of all previous 

 observers, I find the follicular epithelium approximately uniform. 

 The cells are fiat, but extended so as to appear of an unex- 

 pected size in views of the surface of the follicle. This condition 

 does not, however, last very long. A certain number of the cells 

 enlarge considerably, others remaining smaller and fiat. The 

 difi'erences between the larger and the smaller crlls arc more con- 



' ' l);is Ei lici KnoelicnGsclicii.' 

 ' ' Arch. f. Anat. Pliys.,' 1S77. 



