THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



375 



number of columns grow into the stroma, forming the sex cords. In the 

 caudal region there are practically no columns. At first the line of demarka- 

 tion between the cell columns and the stroma is not clearly defined. 



The changes thus far described are common to both sexes and are completed 

 during the fourth or fifth week. The genital ridges or anlagen of the genital 

 glands constitute "indifferent" structures which later become differentiated into 

 either ovaries or testicles. 



Differentiation of the Genital Glands. 



After the fourth or fifth week, certain changes occur in the genital ridges 

 which differ accordingly as the ridges form ovaries or testicles. While the 

 differences are at first not particularly obvious, there are four which become 

 clearer as the changes progress, (i) If the ridge is to become a testicle, the 

 ;ells of the surface epithelium become arranged in a single layer and become 



Rete cords 

 (Rete testis) 



Mesorchium 



Mesothelii 



Tunica 

 albuginea 



> Mesonephros 



Sex cords -. Mr^w.-^V' Glomerulus 

 (convoluted semin- 

 iferous tubules) 



FIG. 327. Transverse section of the left testicle of a pig embryo of 62 mm. 



Bonnet. 



flat. (2) In a developing testicle a layer of dense connective tissue grows be- 

 tween the surface epithelium and the sex cords, forming the tunica albuginea. 



(3) In a tes.ticle there also appears a sharper line of demarkation between the 

 cell columns and the stroma, and the latter shows a more extensive growth. 



(4) Another feature of the testicle is that the sex cells begin to be less con- 

 spicuous and do not increase furth2r in size, but come to resemble the other 

 epithelial elements. The ovarian characters are to a certain extent the oppo- 

 site. (i) The surface epithelium does not become flattened. (2) A layer of 

 connective tissue, corresponding to the albuginea of the testicle, grows be- 



