OF THE VERTEBRATE OVARY. 587 



thelium on the outer side forms the true germinal epithelium, to 

 which the ova are confined (PI. 24, fig. i). In the development 

 of the ovary the stroma becomes differentiated into an external 

 vascular layer, especially developed in the neighbourhood of the 

 germinal epithelium, and an internal lymphatic portion, which 

 forms the main mass of the ovarian ridge (PI. 24, figs. 2, 3, and 6). 



(2) At first the thickened germinal epithelium is sharply 

 separated by a membrane from the subjacent stroma (PL 24, 

 figs, i, 2, and 3), but at about the time when the follicular epi- 

 thelium commences to be formed round the ova, numerous 

 strands of stroma grow into the epithelium, and form a regular 

 network of vascular channels throughout it, and partially isolate 

 individual ova (PI. 24, figs. 7 and 8). At the same time the 

 surface of the epithelium turned towards the stroma becomes 

 irregular (PI. 24, fig. 9), owing to the development of individual 

 ova. In still later stages the stroma ingrowths form a more or 

 less definite tunic close to the surface of the ovary. External 

 to this tunic is the superficial layer of the germinal epithelium, 

 which forms what has been spoken of as the pseudo-epithelium. 

 In many instances the protoplasm of its cells is produced into 

 peculiar fibrous tails which pass into the tunic below. 



(31 Primitive ova. -Certain cells in the epithelium lining 

 the dorsal angle of the body cavity become distinguished as 

 primitive ova by their abundant protoplasm and granular nuclei, 

 at a very early period in development, even before the forma- 

 tion of the genital ridges. Subsequently on the formation of 

 the genital ridges these ova become confined to the thickened 

 germinal epithelium on the outer aspect of the ridges (PL 24, 

 fig. i). 



(4) Conversion of primitive ova into permanent ova 

 Primitive ova may in Scyllium become transformed into perma- 

 nent ova in two ways the difference between the two ways 

 being, however, of secondary importance. 



(a) A nest of primitive ova makes its appearance, either by 

 continued division of a single primitive ovum or otherwise. The 

 bodies of all the ova of the nest fuse together, and a polynuclear 

 mass is formed, which increases in size concomitantly with the 

 division of its nuclei. The nuclei, moreover, pass through a 

 series of transformations. They increase in size and form deli- 



3 8 -2 



