550 THE STRUCTURE AND DEVELOPMENT 



Semper 1 , in his elaborate memoir on the urogenital system of 

 Elasmobranchs, has added very greatly to our knowledge on this 

 subject. In a general way he confirms Ludwig's statements, 

 though he shews that the formation of the ova is somewhat more 

 complicated than Ludwig had imagined. He more especially 

 lays stress on the existence of nests of ova (Ureierernester), 

 derived from the division of a single primitive ovum, and of 

 certain peculiarly modified nuclei, which he compares to spindle 

 nuclei in the act of division. 



My own results agree with those of previous investigators, 

 in attributing to the germinal epithelium the origin both of the 

 follicular epithelium and ova, but include a number of points 

 which I believe to be new, and, perhaps, of some little interest ; 

 they differ, moreover, in many important particulars, both as to 

 the structure and development of the ovary, from the accounts 

 of my predecessors. 



The history of the female generative organs may conveniently 

 be treated under two heads, viz. (i) the history of the ovarian 

 ridge itself, and (2) the history of the ova situated in it. I pro- 

 pose dealing in the first place with the ovarian ridge. 



The Ovarian ridge in Scy ilium. At the stage spoken of in my 

 monograph on Elasmobranch Fishes as stage L, the ovarian ridge 

 has a very small development, and its maximum height is about 

 O'l mm. It exhibits in section a somewhat rounded form, and is 

 slightly constricted along the line of attachment. It presents two 

 surfaces, which are respectively outer and inner, and is formed 

 of a layer of somewhat thickened germinal epithelium separated 

 by a basement membrane from a central core of stroma. The 

 epithelium is far thicker on the outer surface than on the inner, 

 and the primitive ova are entirely confined to the former. The 

 cells of the germinal epithelium are irregularly scattered around 

 the primitive ova, and have not the definite arrangement usually 

 characteristic of epithelial cells. Each of them has a large 

 nucleus, with a deeply staining small nucleolus, and a very scanty 

 protoplasm. In stage N the ovarian ridge has a pointed edge and 

 narrower attachment than in stage L. Its greatest height is 

 about O'l/ mm. There is more stroma, and the basement mem- 

 brane is more distinct than before ; in other respects no changes 



1 Arbeitcn a. d. zool.-zoot. Institut IVurzburg, Bd. n. 



