124 Embr3^onic Development of Ovary and Testis of Mammals 



As stated above^ the albuginea is formed by the transformation of the 

 cells occupying the basal parts of the sex cords, into connective tissue 

 elements, which are liberated l^y the rupture of the membrana propria 

 encasing them (Plate III, Fig. 9). In this manner the sex cords become 

 separated from the peritoneum and undergo further growth and differ- 

 entiation independent of that layer. 



The formation of the mesentery and albuginea and the separation of 

 the sex cords from the peritoneum are far more clearly shown in the pig 

 than in the rabbit, yet I have been able to verify these processes through- 

 out in the latter animal. Coert, 98, has come to essentially the same 

 conclusions, but is cautious in expressing himself in regard to these 

 points, as he well may be, because of the difficulty of following these 

 processes in the rabbit, upon which form he worked. 



The separation of the sex cords from the peritoneum takes place at a 

 slightly earlier period in the female than in the male. Coert, 98, has 

 laid considerable stress upon this fact in the case of the rabbit. How- 

 ever it is not of primary importance in the pig. In the latter animal, 

 it takes place in embryos of 1.6 to 1.7 cm. in length. 



As previously stated, the rete tubules are serially homologous with 

 the sex cords, differing from them at this stage chiefly in the fact that 

 they are less numerous, being isolated from one another by considerable 

 intervals, filled in with connective tissue. The portion of the genital 

 region occupied by the rete tubules becomes elevated to such a aegree 

 as to be quite evident in gross dissections. Coert considers the rete 

 tubules of the rabbit to arise from a mass of unorganized rete blastema 

 by a process of differentiation which slowly progresses inward from the 

 periphery. According to liim, this differentiation process is not com- 

 pleted until after birth. I found these tubules to be distinct and clearly 

 limited in the rabbit embryo of 16 days. This difference between our 

 results may have been due to a difference of technique. Coert used 

 Kleinenberg's picro-sulphuric solution as a fixing agent, while my mate- 

 rial was fixed in Flemming's fluid followed by Heidenhain's iron hsema- 

 toxylin stain. 



Primitive sex cells are present in the rete tubules from the first, 

 but are uncommon, the great mass of cells being similar in character 

 to those of the peritoneum from which they arose. This similarity 

 applies not only to the absence of cell limits in the rete cells of the pig and 

 rabbit, but in the former animal, to the size and staining reaction of the 

 nuclei as w«ll. In the rabbit, these nuclei stain more deeply and are 

 slightly smaller than are the nuclei of the peritoneum and of the germi- 

 natiA^e and follicular cells of the sex cords. As will be seen in a discus- 

 sion of later stages these differences tend to disappear. 



