THE DEVELOPMENT OF THE UROGEXITAL SYSTEM. 



413 



The Ducts of the Genital Glands and the Atrophy of the 

 Mesonephroi. 



In the Female. Strictly speaking, the ovaries are ductless glands; for 

 neither developmentally nor anatomically are the ducts which convey their 

 specific secretion directly connected with them. Furthermore, these ducts are 

 in part transformed into certain organs for the reception and retention of both 

 kinds of sexual elements. In other words, the ducts in part become specially 

 modified to form the vagina and uterus, of which the latter serves as an organ 

 of maintenance for the embryos of the next generation. 



The ducts originate in connection with the mesonephroi, and are known at 

 first as the Mullerian ducts. They appear in both sexes alike but persist only in 

 the female. In the lower Vertebrates they are split off from the mesonephric 

 ducts. In the higher forms, however, their mode of origin is not known with 



Ureter 



Intestine 



Mesonephric duct 



Liver. 



Genital cord 



Mullerian duct 



Left umbilical artery 



Bladder 



Right umbilical artery 



FIG. 371. From a transverse section through the pelvic region of a human embryo 

 of 25 mm. (82-9 weeks). Keibel. 



certainty, but the present evidence favors the view that they arise independ- 

 ently of the mesonephric ducts. They appear in human embryos of 8-14 mm. 

 The mesothelium on the lateral surface of the cephalic end of each mesonephros 

 becomes thickened and then invaginates or dips into the underlying mesen- 

 chyme. By proliferation of the cells at its tip, the invaginated mass grows 

 caudally as a duct parallel with and close to the mesonephric duct. The two 

 ducts come to be embedded in a ridge which at the cephalic end of the meso- 

 nephros is situated laterally, but toward the caudal end bends around and comes 

 to lie ventrally. Beyond (caudal to) the mesonephros the ridge is attached to 

 the lateral body wall, and near the urogenital sinus it meets and fuses with its 

 fellow of the opposite side (Fig. 371). The two Mullerian ducts, contained 

 in the ridges, also approach each other and fuse. The fusion begins in 

 embryos of 25 to 28 mm. (end of second month), and about the same time they 

 open into the dorsal side of the urogenital sinus. The relations of the Mullerian 



