792 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



so-called epithelial nucleus (Felix, '12), is deposited in the genital ridge 

 between the coelomic (germinal) epithelium and the Malpighian (renal) cor- 

 puscles of the mesonephric kidney (fig. 350C). As the epithelial nucleus in- 

 creases in quantity, the genital ridge bulges outward from the general surface 

 of the mesonephric kidney, and, at the same time, the nuclear cells push into 

 the mesonephric substance against the renal corpuscles (figs. 345G; 350A-C). 



During the early stages of the proliferative activities of the germinal epi- 

 thelium in most vertebrates, cellular cords, the sex or medullary cords, appear 

 to arise from the germinal epithelium (fig. 350B). These cords of cells are 

 composed as indicated above of epithelial and germ cells. However, in the 

 mouse and in the human, the proliferative activity of the germinal epithelium 

 is such that the cellular nucleus of the genital ridge arises without a visible, 

 dramatic ingrowth of cellular cords from the germinal epithehum (Brambell, 

 '27; Felix, '12). Still, the cellular sex cords or elongated masses of cells do 

 appear as secondary developments somewhat later in the genital ridges of the 

 mouse and human (fig. 350C). 



The early gonad up to this stage of development represents an indifferent, 

 bipotential condition, having the structural basis for differentiation either into 

 the testis or ovary (see figs. 350C; 351C-3). The indifferent condition in 

 the human sex gland is present when the embryo is about 11 to 14 mm. long, 

 i.e., at about the sixth or seventh week; in the chick, it occurs during the 

 sixth day of incubation; and in the frog, it is present during the larval period. 



2. Development of the Testis 



a. Mammal (Human) 



As the indifferent gonad begins to differentiate into the testis, the following 

 behavior is evident: 



( 1 ) The germinal epithelium becomes a distinct flattened membrane, sep- 

 arated from the primitive tunica albuginea. Unlike the conditions in 

 the developing ovary, the germinal epithelium quickly loses its ger- 

 minative character and forms the relatively inactive, superficial mem- 

 brane of the sex gland (fig. 350D). (The tunica albuginea eventually 

 becomes a connective tissue layer below the coelomic (germinal) 

 epithelium of the male and female sex glands.) 



(2) The primitive sex or medullary cords of the indifferent gonad grow 

 more pronounced, and they possibly may segregate lengthwise into 

 separate, elongated cellular masses (fig. 350D). 



(3) These elongated cellular masses or primitive seminiferous tubules be- 

 come remodeled directly into the later seminiferous tubules. In doing 

 so, their distal ends (i.e., the ends toward the primitive tunica albu- 

 ginea of the sex gland) appear twisted and show anastomoses with 

 neighboring seminiferous tubules, while their proximal ends assume 



