THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 403 



mal part of the urethra. The first evagination appears during the third month. 

 In the male the process continues to form a rather large gland; in the female the 

 structure remains in a rudimentary condition. During the fourth month two 

 evaginations arise from the urethra and develop into the bulbo-urethral 

 (Cowper's) glands in the male, into the larger vestibular (Bartholin's) glands in 

 the female. 



From the course of development it is seen that the epithelium of most of the 

 bladder, of the female urethra and proximal end of the male urethra, of the 



Germinal j|a . Stroma 



epithelium t mm (mesenchyme) 



(mesothelium) 



FIG. 364. Transverse section through the germinal epithelium of a pig embryo of n mm. Nagel. 



The larger cells in the epithelium represent the sex cells, the smaller ones the 



undifferentiated mesothelial cells. 



prostate, of the urogenital sinus, and of the bulbo-urethral and vestibular 

 glands is of entodermal origin. A very small part of the bladder epithelium 

 is of mesodermal origin, since the proximal ends of the mesonephric ducts, 

 which are mesodermal derivatives, are taken up into the wall. All the connec- 

 tive tissue and smooth muscle associated with these organs are derived from 

 the mesoderm (mesenchyme) which surrounds the anlagen. 



THE GENITAL GLANDS. 

 The Germinal Epithelium and Genital Ridge. 



At a very early stage in the formation of the mesonephros, a narrow strip 

 of mesothelium extending along the medial surface becomes thicker and the 

 cells become arranged in several layers (Figs. 314 and 346). The cells become 

 differentiated into two kinds (i) small cuboidal cells with cytoplasm which 

 stains rather intensely, and (2) larger spherical cells with clearer cytoplasm and 



