Bennet Mills Allen .121 



Corpora lutea and masses of interstitial cells are successively forming 

 at the periphery and disintegrating in the interior of the ovary. 



Atretic follicles were found in adult pregnant females of various 

 ages. 



V. DISCUSSION OF RESULTS. 



1. IxDiFFEEENT Stage. — TliB genital ridge first appears as an area 

 of thickened peritoneum and underlying mesenchyme (stroma), extend- 

 ing the entire length of the mesonephros, and situated on the ventro- 

 median face of that organ. The tissues composing it are in no wise dif- 

 ferent from those forming the remainder of the investment of the 

 mesonephros. In section, the peritoneum is found to be separated from 

 the stroma by a more or less distinct basement membrane formed by 

 the interlacing of protoplasmic fibrils proceeding from the nuclei of 

 peritoneal and stroma cells (Plate I, Figs. 2 and 3). 



The cells composing the peritoneum and stroma tissues are almost 

 wholly without evident boundaries. Only here and there does one find 

 scattered cells with distinct cell boundaries, centrosphere, centrosome, 

 large nucleus, and clear c^-toplasm — the so-called primitive sex cells. 

 They occur in all parts of the genital ridge Init are most numerous in 

 that region in which the sex gland will form. 



The distinction between peritoneum and stroma is not based upon any 

 essential difference in the character of their component cells at this 

 early period, but is based upon their arrangement, the nuclei of the 

 peritoneum being arranged with their long axes parallel to one another 

 and perpendicular to the basement membrane, while those of the stroma 

 tissue lie with their long axes usually parallel to the basement 

 membrane of the peritoneum which is very faint at certain points where 

 active division of the peritoneal cells is taking place. This is due to the 

 fact that peritoneal nuclei are being crowded through the membrane 

 by mutual pressure, caused by their rapid multiplication. 



In the 10 cm. stage, a regional differentiation begins to appear in 

 the genital ridge. This is marked by the formation of numerous crowded 

 peritoneal invaginations (Plate I, Fig. 4) in the middle third; less 

 numerous and deeper invaginations in the anterior third; and the almost 

 total lack of them in the posterior third. The regions from front to 

 rear, as thus marked off, are the rudiments of the rete, sex gland and 

 mesenteric ridge, respectively. 



These invaginations are caused by a progressive multiplication of the 

 peritoneal nuclei. Although the first formation of these cords is a true 

 process of invagination, further gi'owth is centrifugal, the peritoneum 

 10 



