OF THE VERTEBRATE OVARY. 577 



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 shewn in the surface view on PI. 25, 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 grow- 

 ing to the colossal size which it eventually attains, the follicular 

 epithelium does not to any great extent alter in constitution. 

 It grows thicker on the whole, and as the vitelline membrane 

 gradually atrophies, its lower surface becomes irregular, exhibit- 

 ing somewhat flattened prominences, which project into the 

 yolk. At the greatest height of the prominences the epithelium 

 may reach a thickness of O'o6 mm., or even more. The arrange- 

 ment of the tissues external to the follicular epithelium is the 

 same in Raja as in Scyllium. 



The most interesting point connected with the follicle, both 

 in Scyllium and Raja and presumably in other Elasmobranchs 

 is that its epithelium at the time when the egg is rapidly ap- 

 proaching maturity is composed with more or less of distinctness 

 of two forms of cells. One of these is large flask-shaped and rich 

 in protoplasm, the other is small, consisting of a mere film of 

 protoplasm round a nucleus. Considering that the larger cells 

 appear at the time of rapid growth, it is natural to interpret 

 their presence as connected with the nutrition of the ovum. 

 This view is supported by the observations of Eimer and Braun, 

 on the development of Reptilian ova. In many Reptilian ova 

 it appears from Eimer's 1 observations, that the follicular epi- 

 thelium becomes several layers thick, and that a differentiation 

 of the cells, similar to that in Elasmobranchs, takes place. The 

 flask-shaped cells eventually undergo peculiar changes, becoming 

 converted into a kind of beaker-cell, with prolongations through 

 the egg membranes, which take the place of canals leading to 

 the interior of the egg. Braun also expresses himself strongly 

 in favour of the flask-shaped cells functioning in the nutrition of 

 the egg s . That these cells in the Reptilian ova really corre- 



1 Archiv f. mikr. Anat. Vol. vin. 



z Braun, " Urogeuitalsystem d. Amphibien," Arbeiten a. d. zool.-zoot. Institut 



