CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS 



215 



urethral anlage enlarges and forms the bladder and a portion of the urethra. In 

 7 mm. embryos the proximal ends of the mesonephric ducts are funnel-shaped, 

 and at 10 mm., with the enlargement of the bladder, these ends are taken up into 



Allantols 



Rectum 



Mesonephric duels 



Vesica-urethral 

 anlaoe 



Phallic portion 

 of urogenital sinus 



Mefanephros 



Ureter 



Fig. 211. — Reconstruction of the caudal portion of an 11.5 mm. embryo showing the differentiation of 

 the rectum, bladder and urethra (Keibel). 



Genital gland 

 Mesonephric fold 

 /Inlaae of bladder 



Uhro-Vay'tnal anlaye 



Phallic urethra. 

 Cloaca! membrane 



Mesonephros 

 Mesonephric duct 



Ureter 

 Muellenan duct 



Mesonephric duct 



Ttectum 



Spinal cord 



Fig. 212. — Reconstruction of the caudal end of a 25 mm. embryo showing the complete separation of the 

 rectum and urogenital sinus and the relations of the urogenital ducts (after Keibel's model). 



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- 



