INTERNAL SEXUAL TRANSFORMATIONS TESTIS 



213 



duct. The solid end of the tube grows caudalward beneath the epithelium, 

 lateral to the mesonephric, or male ducts (Pigs. 223 to 225). Eventually, 

 by way of the genital cord, the Miillerian ducts reach the median dorsal 

 wall of the urogenital sinus and open into it (Figs. 219 and 238 A). Their 

 further development into uterine tubes, uterus, and vagina is described 

 on page 220. Embryos not longer than 12 mm. are thus characterized by 

 the possession of indifferent genital glands and both male and female 

 genital ducts. There is as yet no sexual differentiation. The develop- 

 ment and position of the Mullerian ducts is well shown in ventral dissec- 

 tions of pig embryos (Figs. 223 and 224); the mesonephroi of the pig are 

 much larger than in man. In the lowest vertebrates the Mullerian duct 

 arise by a longitudinal splitting of the mesonephric duct. 



Ligamentum testi 



Mesonephric tubule 

 Mesorchium 

 Anlage of rele testis 



Mesonephric duct 



Miillerian duct 

 Intermediate cord 

 Testis cord 



Epithelium 

 Tunica albuginea 



FIG. 225. Transverse section through the left testis and mesonephros of a 20 mm. human 



embryo. X 250. 



B. INTERNAL SEXUAL TRANSFORMATIONS 



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

 genital glands show two characters which mark them as testes: (i) the 

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

 the occurrence between epithelium and testis cords of a layer of tissue, the 

 anlage of the tunica albuginea (Fig. 225). According to Felix (1912), the 

 testis cords of man are developed suddenly from the loose, inner epithelial 

 mass by a condensation of its cells. The cords converge and grow smaller 

 .towards the mesorchium, where they form the dense, epithelial anla'ge 



