STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARY. 407 



spicuous in sections than in surface views, and though the distri- 

 bution of the cells is somewhat irregular, it may still be pre- 

 dicted as an almost invariable rule that the smaller cells of the 

 follicle vrill line that part of the surface of the ovum, near to which 

 the germinal vesicle is situated. On PI. XIX, fig. 30, is shown 

 in section a fairly average arrangement of the follicle cells. 

 Semper considers the larger cells of such a follicle to be probably 

 primitive ova destined to become permanent ova. This view I 

 cannot accept : firstly, because these cells only agree with primitive 

 ova in being exceptionally large — the character of their nucleus, 

 with its large nucleolus, being not very like that of a primitive 

 ovum. Secondly, becausethey shadeinto ordinary cells of the follicle; 

 and thirdly, because no evidence of their becoming ova has come 

 before me, but rather the reverse, in that it seems probable that 

 they have a definite function connected with the nutrition of the 

 egg. To this point I shall return. 



In the next stage the small cells have become still smaller. 

 They are columnar, and are wedged in between the larger ones. 

 No great regularity in distribution is as yet attained (PL XIX, 

 fig. 81). Such a regularity appears in a later stage (PI. XIX, 

 fig. 32), which clearly corresponds with fig. 8 on PL XXXI V 

 of Schultz's paper, and also with the stage of Scyllium in PL XIX, 

 fig. 29, though the distinction between the two kinds of cells is 

 here far better marked than in Scyllium. The big cells have now 

 become flask-shaped like those in Scyllium, and send a pro- 

 cess down to the vitelline membrane. The smaller cells are 

 arranged in two or three tiers, but the larger cells in a single 

 layer. The distribution of the larger and smaller cells is in some 

 instances very regular, as shown in the surface view on PL XIX, 

 fig. 33. There can, it appears to me, be no doubt that Schultz's 

 View of the smaller cells being lymph-cells which have migrated 

 into the follicle cannot be maintained. 



The thickness of the epithelium at this stage is about 0"04 

 mm. In the succeeding stages, during which the egg is rapidly 

 growing to the colossal size which it eventually attains, the fol- 

 licular epithelium does not to any great extent alter in constitu- 

 tion. It grows thicker on the whole, and as the vitelline mem- 

 brane gradually atrophies, its lower surface becomes irregular, 

 exhibiting somewhat flattened prominences, which project into 

 the yolk. At the greatest height of the prominences the epi- 

 tlu'Hum may reach a thickness of 0'06 mm., or even more. 

 The arrangement of the tissues external to the follicular epi- 

 thelium is the same in Eaja as in Scyllium. 



The most interesting point connected with the follicle, both in 

 Scyllium and Haja and presumably in other Elasmobranchs is that 

 its epithelium at the time when the egg is rapidly approaching 



