212 



THE UEOGENITAL SYSTEM 



ducts reach the median dorsal wall of the urogenital sinus and open into it (Figs. 

 2 19 and 238 A). Their further development into uterine tubes, uterus, and vagina 

 is described on page 219. Embryos not longer than 12 mm. are thus character- 

 ized by the possession of indifferent genital glands and of both male and female 

 genital ducts. There is as yet no sexual differentiation. The development and 

 position of the Miillerian ducts is well shown in ventral dissections of pig em- 

 bryos (Figs. 223 and 224) ; the mesonephroi of the pig are of enormous size. 

 In the lowest vertebrates the Miillerian duct arises by a longitudinal splitting of 

 the mesonephric duct. 



Mesenli 



: tubule 

 Mesorchium 

 Anlage of rele testis 



Mesonephric duct 



Miillerian duct 



Intermediate cord 



Epithelium 

 Tunica aibuginea 



Fig. 225.— Transverse section through the left testis and mesonephros of a 20 mm. human embryo. 



X 250. 



Differentiation of the Testis.— In the male embryos of 13 mm. the genital 

 glands show two characters which mark them as testes: (1) the occurrence of 

 branched, anastomosing cords of cells, the testis cords; (2) the occurrence between 

 epithehum and testis cords of a layer of tissue, the anlage of the ttinica aibuginea 

 (Fig. 225). According to Fehx, the testis cords are developed suddenly from the 

 loose, inner epithelial mass by a condensation of its cells. The cords converge 

 and grow smaller towards the mesorchiurh, where they form the dense, epi- 

 theUal anlage of the rete testis. Two or three layers of loosely arranged cells 

 between the testis cords and the epithelium constitute the anlage of the tunica 

 aibuginea. According to Allen (Amer. Jour. Anat., vol. 3, 1904), the testis cords 



