56 BENNET M. ALLEN. 



The earliest rudiment of the sex gland is situated in the genital 

 ridge, which consists of a zone of thickened peritoneum, running 

 the entire length of the mesonephros, parallel and close to the 

 mesentery which unites the latter to the body wall. The rete is 

 formed from the anterior part of the genital ridge and extends, in 

 the pig, from about the region of the sixth glomerulus to a point 

 about opposite the twentieth glomerulus, as shown in a number 

 of series. However, these limits are variable. In the rabbit the 

 limits are more constant, but still variable, the rete extending 

 approximately from the sixth to the twelfth glomerulus. The 

 anterior end of the sex gland rudiment slightly overlaps the pos- 

 terior end of the rete region. Posterior to the sex gland is a 

 section of the genital ridge that does not develop beyond the very 

 early formation of a region of thickened peritoneum. In each of 

 these three zones of the genital ridge are found scattered cells 

 with distinct cell walls, clear cytoplasm, large round nucleus, 

 centrosphere and centrosome the so-called primitive sex cells. 



In its early stages of development the genital ridge consists of 

 a thickened layer of peritoneum overlying a loose mesenchyma ; 

 the cells of the latter are to all appearances identical with those 

 of the peritoneum, from which they undoubtedly originate. This 

 resemblance applies both to the character of their nuclei and to 

 their lack of definite cell boundaries. The peritoneal layer and 

 underlying mesenchyma are separated from one another by the 

 basement membrane of the former. This is formed by the inter- 

 lacing of protoplasmic fibrils given off by the cells of both layers. 

 In later stages, one finds a progressive crowding of the peritoneal 

 nuclei. In the rete and sex gland regions this results in the for- 

 mation of tubular peritoneal invaginations, which are limited from 

 the surrounding mesenchyma by the persistent membrana propria. 

 In the sex gland region these tubular cords of cells may be known 

 as the sex-cords. At this indifferent stage they are closely massed 

 together side by side, and the very narrow interspaces between 

 them contain scattered mesenchyme cells, which from now on, 

 may be considered under the general term of stroma. This is 

 used to designate the loose connective tissue of both ovary and 

 testis. Invaginations, in all respects similar to these sex-cords, 

 arise from the peritoneum of the rete region. They lie further 



