THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 381 



mains smaller. In both cases, however, the histological changes are essentially 

 the same. 



The Testicle. The processes that give rise to the "indifferent" genital 

 glands have been described (p. 373 et seq.}. It has also been stated that there 

 appears during the fourth or fifth week a structure that forms one of the char- 

 acteristic features of the testicle. This is a layer of dense connective tissue 

 which develops beneath the surface epithelium and constitutes the tunica 

 albuginea (p. 375), and which separates the surface epithelium from the sex 

 cords (Fig. 327) . The epithelium becomes reduced to a single layer of flat cells, 

 although the cells on the tip of the gland usually remain high until after birth. 

 Naturally this epithelium is continuous around the hilus of the testicle with the 

 epithelium (mesothelium) of the abdominal cavity. Within the gland are the 

 sex cords the progenitors of the convoluted seminiferous tubules, which become 

 quite distinctly marked off from the stroma by a basement membrane. In the 



Interstitial cell Sex cell 



i 



t^ilfcM^ 



r _ > ~^ . ... j%j1 __ 



&$ - " ; -t^*!^; 

 i^n^^^ai^ 



I. t vv v : ... a -,--?V*- &* i- -"** ~it~;"tja--~~ > 



'^8lf@^ 



Mesothelium Tunica Supporting cell 



albuginea (of Sertoli) 



FIG. 332. From a section of the testicle of a human foetus of 35 mm., showing a developing 

 convoluted seminiferous tubule. Meyer-Ruegg, Biihler. 



hilus region lie the rete cords the progenitors of the rete testis and the straight 

 seminiferous tubules (Fig. 327) . The rete cords of the testicle are homologues of 

 the rete cords of the ovary, and are derivatives of the germinal epithelium on the 

 cephalic portion of the "indifferent" gland (p. 374). 



The sex cords at first are solid masses composed of several layers of cells. 

 The latter are of two kinds, as in the ovary (i) smaller, darkly staining indiffer- 

 ent cells, and (2) larger, clearer sex cells (Fig. 332). The sex cells lose their 

 clearness and come to resemble again the undifferentiated epithelial cells. 

 They represent the spermatogonia, which correspond to the primitive ova. 

 The spermatogonia proliferate very rapidly and become much more numerous 

 than the epithelial cells. The sex cords become more and more coiled during 

 development and anastomose with one another near the convex surface 

 of the testicle. Beginning after birth and continuing up to the time of 

 puberty, lumina appear in them by displacement of the central cells, and 



