2l8 



UROGENITAL SYSTEM 



sexes there also develop a pair of female ducts (of Mueller) . In embryos of 10 to 

 ii mm. the Muellerian ducts develop as ventro-lateral thickenings of the uro- 

 genital epithelium at the level of the third thoracic segment and near the cranial 

 ends of the mesonephroi. Next, a ventro-lateral groove appears in the epi- 

 thelium of the mesonephric fold (Fig. 215 A). Caudally, the dorsal and ventral 

 lips of the groove close and form a tube which separates from and lies beneath the 

 epithelium (Fig. 215 B). Cranially, the tube remains open as the funnel-shaped 

 ostium abdominale of the Muellerian duct. The solid end of the tube grows 

 caudalward beneath the epithelium, lateral to the mesonephric or male ducts 



A 



Lateral body wall 



Mueller/ an groove ^sss.'^sss:.imi^sf^ e ~^fi^ ^Sfc. n/l 



/VlesenT&ry 



Mesonephric 

 tubuLe. 



GenitaL cjland 





Anlacje. of 

 Muellerian duct 



FIG. 215. Transverse sections through the anlage of the right Muellerian duct from a 10 mm. 

 embryo. A, showing the groove in the urogenital epithelium; B, three sections caudad showing the 

 tubular anlage of the duct. X 250. 



(Figs. 216 and 218). Eventually, by way of the genital cord, the Muellerian 

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

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

 page 226. Embryos not longer than 12 mm. are thus characterized by the pos- 

 session 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 

 Muellerian ducts is well shown in ventral dissections of pig embryos (Figs. 216 

 and 217). Note the enormous size of the mesonephroi. 



Differentiation of the Testis. In male embryos of 13 to 15 mm. the genital 



