2l6 



UROGENITAL SYSTEM 



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 into the dorsal wall of the urethra (Fig. 212). 

 The fate of the phallic portion of the urogenital sinus is described on p. 234 in 

 connection with the external genitalia. 



The allantois between the bladder and the umbilicus is known as the urachus. Usually 

 the epithelium of the urachus degenerates, but portions may persist and produce cysts. In 

 some cases it forms after birth a patent tube opening at the umbilicus. Its connective tissue 

 layers always persist as the fibrous lig. vesico-umbilicale medium. 



The transitional epithelium of the bladder appears at 60 mm. The outer longitudinal 

 layer of small 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. and the sphincter vesicas 

 in embryos of 90 mm. 



Mesencephalon 



Rhombencephalon 



R. lung _k. 



Mesonephric fold 



Lower extremity 



Prosencephalon 



Heart 



Esophagus 



Genital fold 



Genital eminence 



Tail 



Fig. 213. — Ventral view of the urogenital folds in a human embryo of five weeks (Kollmann's Atlas). 



THE GENITAL GLANDS AND DUCTS— INDIFFERENT STAGE 



As to origin and early development, the ovary and testis are alike. The 

 urogenital fold (see p. 205) is the anlage of both the mesonephros and the genital 

 gland (Fig. 213). At first two-layered, its epithelium in embryos of 5 mm. thick- 



