OF THE VERTEBRATE OVARY. 589 



growth. The first formed and outer of these arises in some 

 instances before the formation of the follicular epithelium, and 

 would seem to be of the nature of a vitelline membrane. The 

 inner one is the zona radiata with a typical radiately striated 

 structure. It is formed from the vitellus at a much later period 

 than the proper vitelline membrane. It is more developed in 

 Scyllium than in Raja, but atrophies early in both genera. By 

 the time the ovum is nearly ripe both membranes are very much 

 reduced, and when the egg (in Scyllium and Pristiurus) is laid, 

 no trace of any membrane is visible. 



(7) The vitellus. The vitellus is at first faintly granular, 

 but at a later period exhibits a very distinct (protoplasmic) 

 network of fibres, which is still present after the ovum has been 

 laid. 



The yolk arises, in the manner described by Gegenbaur, in 

 ova of about O'o6 mm. as a layer of fine granules, which stain 

 deeply with osmic acid. They are at first confined to a stratum 

 of protoplasm slightly below the surface of the ovum, and are 

 most numerous at the pole furthest removed from the germinal 

 vesicle. They are not regularly distributed, but are aggregated 

 in small masses. They gradually grow into vesicles, in the inte- 

 rior of which oval solid bodies are developed, which form the 

 permanent yolk-spheres. These oval bodies in the later stages 

 exhibit a remarkable segmentation into plates, which gives them 

 a peculiar appearance of transverse striation. 



Certain bodies of unknown function are occasionally met 

 with in the vitellus, of which the most remarkable are those 

 figured at x on PL 25, fig. 25 A. 



(8) The germinal vesicle. A reticulum is very conspicuous 

 in the germinal vesicle in the freshly formed ova, but becomes 

 much less so in older ova, and assumes, moreover, a granular 

 appearance. At first one to three nucleoli are present, but they 

 gradually increase in number as the germinal vesicle grows 

 older, and are frequently situated in close proximity to the 

 membrane. 



