THE GENITAL GLANDS AND DUCTS INDIFFERENT STAGE 2CX) 



nephric 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 meso- 

 nephric ducts. The lateral walls of the bladder anlage grow more rapidly 

 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 hillock, Mailer's tubercle, into 

 the dorsal wall of the urethra (Fig. 219). 



Thus a triangular area, roughly bounded by the openings of the ureters and 

 ejaculatory ducts, is of mesodermal origin. The narrowed apex of the bladder, con- 

 tinuous with the allantoic stalk at the umbilicus, is known as the urachus. It persists 

 as the solid, fibrous ligamenlum umbilicale medium. Contrary to earlier views, the 

 allantois contributes nothing to the bladder or urachus (Felix, 1912). 



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 musclelayer is found at 55 mm. 

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



Anomalies. A conspicuous malformation is that of a persistent cloaca, due to the 

 failure of the rectum and urogenital sinus to separate. The bladder sometimes opens 

 widely onto the ventral body wall and is everted through the fissure ; a urogenital aper- 

 ture corresponding to the upper extent of the primitive cloacal membrane (Fig. 216, C, D) 

 would cause this condition. At times, the urachus remains a patent tube, opening at the 

 umbilicus. Portions of its epithelium which fail to degenerate may form cysts. 



THE GENITAL GLANDS AND DUCTS 

 A. INDIFFERENT STAGE 



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

 The urogenital fold (p. 198) 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 ccelom ventrally, producing 

 the longitudinal genital fold (Fig. 208). The genital fold thus lies mesial 

 and parallel to the mesonephric fold. Large primordial germ 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 mesenteric epithelium 

 and thence into the epithelium of the genital fold. It is probable that the 

 definitive germ cells of the genital glands are descendants of these ele- 

 ments. At 10 to 12 mm. the genital anlage 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 meta- 

 nephroi, 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 

 u 



