794 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



the compartment and the tubuli recti lying proximally toward the rete 

 testis and mediastinum. 



The formation of the rete-testis canals and of the urogenital union in general 

 has been the subject of much controversy. In the elasmobranch fishes, Brachet 

 ('21) considered the rete-testis canals to be formed by the nephrostomial 

 canals of the anterior mesonephric tubules which unite with the developing 

 seminiferous tubules. *In the frog, Witschi ('21) believed a condensation of 

 cells in the hilus of the testis formed the rudiments of the rete tubules and 

 that these rudiments unite with the mediastinal ends of the seminiferous 

 tubules on the one hand and with the renal corpuscles of the mesonephric 

 tubules on the other, forming the urogenital union. In the chick, it is pos- 

 sible that the rete tubules arise as outgrowths from the renal corpuscles (Lillie, 

 '30, p. 394). In the human, Felix ('12) concluded that the rete tubules arise 

 from a rete rudiment in the testicular hilus, but de Winiwarter ('10) con- 

 sidered them as outgrowths from the renal (Malpighian) corpuscles of the 

 mesonephric tubules. 



b. Chick 



The development of the testis in the chick closely resembles that described 

 above for the mammal. The sex or medullary cords arise during the fifth and 

 sixth days of incubation from the germinal epithelium (fig. 350B). For a de- 

 tailed description, consult Swift, '16, and Lillie, '30. 



c. Frog 



The main essentials of testicular development in the frog follow the pattern 

 described above. However, because the gonadal rudiment of the frog differs 

 slightly from that described for the mammal, certain features are presented 

 here. 



The germinal epithelium of the primitive gonad of the anuran is thin, and 

 the primitive germ cells lie, together with various epithelial elements, below the 

 germinal epithelium. In the center of this primitive gonad is the slit-like primi- 

 tive gonadal cavity. This cavity is surrounded by the germ cells, epithelial 

 cells and germinal epithelium. This condition may be regarded as the indifferent 

 stage of gonadal development. 



In the differentiation of the testis, cellular strands, the rudiments of the 

 future rete tubules, grow down into the primitive gonadal cavity from the 

 mesonephric kidney. In the male, these mesonephric strands are thick and 

 grow rapidly. The primitive germ cells and epithelial cells eventually grow 

 inward across the primitive gonadal cavity and become clustered about the 

 mesonephric strands (fig. 350G). 



At first the germ cells and epithelial elements form cellular nests associated 

 with the mesonephric strands. Later, the cellular nests and associated cells 

 from the mesonephric strands elongate into the primitive seminiferous tubules. 



