404 TEXT-BOOK OF EMBRYOLOGY. 



slightly cranial and lateral to the former. (Compare Figs. 362 and 363.) This 

 condition is reached in embryos of 12 to 14 mm. The point at which these two 

 sets of ducts open marks the boundary between a slightly larger cephalic part 

 of the sinus, the anlage of the bladder, and a smaller caudal part which becomes 

 the urethra and urogenital sinus (Fig. 363). 



After the second month the bladder becomes larger and more sac-like, and 

 the openings of the ureters migrate farther cranially to their final position. The 

 lumen of the bladder is at first continuous with the lumen of the allantoic duct, 

 but the duct degenerates into a solid cord of cells, the urachus. The latter 

 degenerates still further and finally remains only as the middle umbilical liga- 



Urorectal fold Mesonephric duct 



Kidney bud. 



Urachus 



Cloaca <^ 



Urogenital sinus 



,__ ,^___, . ,-_ Rectum 

 Cloacal membrane 



:audal gut 



FIG. 361. From a model of the cloacal region of a human embryo slightly older than 



that shown in Fig. 360. Keibel. 



The arrow points to the developing partition (urorectal fold) between the rectum and urogenital 

 sinus. The opening of the mesonephric duct into the urogenital sinus is indicated by a 

 small seeker. 



ment. It seems quite probable that the bladder is derived almost wholly from 

 the cloaca. A small part arises from the inclusion of the ends of the mesoneph- 

 ric ducts. If any part is derived from the allantoic duct, it is only the apex. 

 After the bladder begins to enlarge, the adjacent portion of the urogenital 

 sinus becomes slightly constricted. This marks the beginning of the urethra. 

 In the female the constricted part represents practically the entire urethra. 

 In the male it represents only the proximal end, the other portion developing 

 in connection with the penis (p. 428). The urogenital sinus is narrow and 

 tubular at its junction with the urethra; more distally it is wider and is shut off 

 from the exterior by the cloacal membrane. After the embryo reaches a length 

 of 1 6 to 17 mm., the membrane ruptures and the sinus opens on the surface. 



