CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS 213 



includes both the ascending and descending limbs of Henle's loop and a portion 

 of the proximal convoluted tubule. Henle's loop is differentiated during the 

 fourth fetal month (Toldt) and extends from the pars radiata of the cortex into 

 the medulla (Fig. 207). The concavity of Bowman's capsule, into which grow 

 the arterial loops of the glomerulus, is at first shallow. Eventually the walls of 

 the capsule grow about and enclose the vascular knot, except at the point where 

 the arteries enter and emerge (Fig. 205, 4 and 5). Renal corpuscles are first 

 fully formed in 28 to 30 mm. embryos. The new corpuscles are formed peripher- 

 ally from persisting nephrogenic tissue, hence in the adult the oldest corpuscles 

 are those next the medulla. Reconstructions of the various stages in the de- 

 velopment of the uriniferous tubules are shown in Fig. 208. 



Renal Arteries. — One or more of the mesonephnc arteries is transformed into the 

 renal artery of the metanephros. As any one of the mesonephric arteries may thus form the 

 renal artery, and as they anastomose, the variation of the renal vessels both as to position and 

 number is accounted for. 



Anomalies. — If the uriniferous tubules fail to unite with the collecting tubules, cystic 

 degeneration may take place. The cystic kidneys of pathology may thus be produced. The 

 nephrogenic tissue of the paired kidney anlages may fuse, resulting in the union of their cortex. 

 Double or triple ureters are sometimes present. 



DIFFERENTIATION OF CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS 



In embryos of 3 to 4 mm. the cloaca, a caudal expansion of the hind-gut, is 

 in contact ventrally with the ectoderm, and ectoderm and entoderm together 

 form the cloacal membrane (Fig. 209 A). Ventro-cranially the cloaca gives off 

 the allantoic stalk. At a somewhat later stage, the cloaca receives laterally the 

 mesonephric ducts and caudally is prolonged as the tail-gut (Fig. 209 B) . 



In embryos of 5 mm. the ureteric anlages of the metanephroi are present as 

 buds of the mesonephric ducts (Fig. 209 C, D). Next, the saddle-like partition 

 wall between the intestine and allantois grows caudally, dividing the cloaca 

 into a dorsal rectum and ventral primitive urogenital sinus. The division is 

 complete in embryos of 11 to 15 mm. and at the same time the partition, fusing 

 with the cloacal membrane, divides it into the anal membrane of the gut and the 

 urogenital membrane. At 11 mm., according to Felix, the primitive urogenital 

 sinus by elongation and constriction is differentiated into two regions: (1) a 

 dorsal vesico-urethral anlage which receives the allantois and mesonephric duct, 

 and is connected by the constricted portion with (2) the phallic portion of the 

 urogenital sinus (Figs. 210 and 211). The latter extends into the phallus of 

 both sexes and forms a greater part of the urethra (Fig. 212). The vesico- 



