2o8 THE UROGENITAL SYSTEM 



its wall until the ureters and mesonephric ducts acquire separate openings. The 

 ureters, having previously shifted their openings into the mesonephric ducts from 

 a dorsal to lateral position, now open into the vesico-urethral anlage lateral to 

 the mesonephric ducts. The lateral walls of the bladder anlage grow more rap- 

 idly than its dorso-median urethral wall, hence the ureters are carried cranially 

 and laterally upon the wall of the bladder, while the mesonephric ducts, now the 

 male ducts, open close together on a hiUock, Miiller's tubercle, into the dorsal 

 wall of the urethra (Fig. 219). The fate of the phalHc portion of the urogenital 

 sinus is described on p. 226 in connection with the external genitaha. 



The apex of the bladder, continuous with the allantoic stalk at the umbilicus, is known 

 as the urachus. Usually the epitheUum of the urachus degenerates, but portions may persist 

 and produce cysts. In some cases it forms after birth a patent tube opening at the umbihcus. 

 Its connective tissue layers always persist- as the fibrous lig. umbilicale medium. 



The transitional epithelium of the bladder appears at 60 mm. (C H). The outer 

 longitudinal layer of smooth muscle develops in 22 mm. embryos, and, in 26 mm. embryos, 

 the circular muscle appears. The inner longitudinal muscle layer is found at 55 mm. (C H) 

 and the sphincter vesicae in fetuses of 90 mm. (C H). 



Anomaly.— A conspicuous malformation is that of a persistent cloaca, due to the failure 

 of the rectum and urogenital sinus to separate. 



THE GENITAL GLANDS AND DUCTS— INDIFFERENT STAGE 

 In origin and early development, the ovary and testis are identical. The 

 urogenital fold (p. 197) is the anlage of both the mesonephros and the genital 

 gland (Figs. 122 and 220). At first two-layered, its epithelium in embryos of 

 5 mm. thickens over the ventro-median surface of the fold, becomes many- 

 layered, and bulges into the coelom ventrally, producing the longitudinal genital 

 fold. The genital fold thus Hes mesial and parallel to the inesonephric fold. 

 Large primitive sex cells are found in 2.5 mm. embryos in the entoderm of the 

 future intestinal tract (Fuss). At 3.5 mm. they migrate into the dorsal mesen- 

 teric epithelium and thence into the epithelium of the genital fold. At 10 to 

 12 mm. the genital epithelium shows no sexual differentiation (Fig. 221). There 

 is a superficial epithelial layer and an inner epithelial mass of somewhat open 

 structure. 



Owing to the great development of the suprarenal glands and metanephroi, 

 the cranial portions of the urogenital folds, at first parallel and close together, are 

 displaced laterally. This produces a double bend in each fold which, in 20 mm. 

 embryos, shows a cranial longitudinal portion, a transverse middle portion between 

 the bends, and a longitudinal caudal portion. In the last named segment, the 

 mesonephric ducts course to the cloaca and here the right and left folds fuse, 



