THE GENITAL GLANDS AND DUCTS INDIFFERENT STAGE 



211 



transverse middle portion between the bends, and a longitudinal caudal 

 portion (Fig. 238 A). In the last named segment, the mesonephric ducts 

 course to the cloaca, and here the right and left folds fuse, producing the 

 genital cord (Fig. 232). As the genital glands increase in size they become 

 constricted from the mesonephric fold by lateral and mesial grooves until 

 the originally broad base of the genital fold is converted into a stalk (Figs. 

 225 to 227). This stalk-like attachment extends lengthwise and forms in 

 the male the mesorchium, in the female the mesovarium. The urogenital 

 fold is, at the same time, constricted from the dorsal body wall until it is 

 attached only by a narrow mesentery which eventually forms either the 

 ligatnentum testis or ligamentum ovarii. 



Lateral body wall 



Miillerian groove 



- Mesentery 



Mesonephric tubule 



Genital gland 



Anlage of Miillerian duct 



FIG. 222. Transverse sections through the anlage of the right Mullerian duct from a 10 

 mm. human embryo. X 250. A, showing the groove in the urogenital epithelium; B, three 

 sections caudad, showing the tubular anlage of the duct. 



Indifferent Stage of the Genital Ducts. The mesonephric ducts, with 

 the degeneration of the mesonephroi, become the male genital ducts. In 

 both sexes there also develop a pair of female ducts (of Muller). In em- 

 bryos of 10 mm. these Mullerian ducts develop as ventro-lateral thickenings 

 of the urogenital 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 epithelium of the mesonephric fold (Fig. 222 A). Caudally, 

 the dorsal and ventral lips of the groove close and form a tube which sepa- 

 rates from the epithelium and lies beneath it (Fig. 222 B). Cranially, the 

 tube remains open as the funnel-shaped ostium abdominale of the Mullerian 



