THE DEVELOPMENT OF THE UROGENITAL SYSTEM. 



403 



THE URINARY BLADDER, URETHRA AND UROGENITAL SINUS. 



As described elsewhere, the allantois appears at an early stage as an evagi- 

 nation from the ventral side of the caudal end of the primitive gut (Fig. 282), 

 grows out into the belly stalk, and finally becomes enclosed in the umbilical cord 

 (p. 118). As the embryo develops, the proximal end of the allantois becomes 

 elongated to form a stalk or duct which extends from the caudal end of the 

 gut to the umbilicus (Fig. 285). The portion of the gut immediately caudal to 

 the attachment of the allantoic duct becomes dilated to form the cloaca which 

 at first is a blind sac, its cavity being separated from the outer surface of the 

 embryo by the cloacal membrane (Fig. 360) . The latter is composed of a layer of 

 entoderm and a layer of ectoderm, with a thin layer of mesoderm between. The 

 cloaca then becomes separated into two parts a larger ventral part which forms 



Mesoneohric duct 



Intestine Kidney bud 

 f\\ 



Cloacal membrane 



Caudal gut 



Notochord 



Neural tube 



FIG. 360. From a model of the cloaca and the surrounding structures in a 

 human embryo of 6.5 mm. Keibel. 



, 



the urogenital sinus and a smaller dorsal part which forms the rectum. This 

 is accomplished by a fold or ridge which grows from the lateral wall into the 

 lumen and meets and fuses with its fellow of the opposite side. The fusion be- 

 gins at the cephalic end, in the angle between the allantoic duct and the gut, 

 and gradually proceeds caudally until the separation is complete as far as the 

 cloacal membrane. The mass of tissue forming the partition is called the uro- 

 rectalfold (Fig. 361). The openings of the mesonephric ducts, which primarily 

 were situated in the lateral cloacal wall (p. 392), are situated after the separation 

 in the dorso-lateral wall of the urogenital sinus (compare Figs. 360, 361, 362). 

 During the separation of the urogenital sinus from the rectum, certain 

 changes take place in the proximal ends of the mesonephric ducts and ureters. 

 The ends of the ducts become dilated and are gradually taken up into the wall of 

 the sinus. This process of absorption continues until the ends of the ureters are 

 included, with the result that the ducts and ureters open separately, the latter 



