CLOACA, BLADDER, URETHRA AND UROGENITAL SINUS 205 



itive loop of Stoerck includes both the ascending and descending limbs of Henle's 

 loop and a portion of the proximal convoluted tubule. Henle's loop is differ- 

 entiated during the fourth fetal month (Toldt) and extends from the pars radiata 

 of the cortex into the medulla (Fig. 214). The concavity of Bowman's capsule, 

 into which grow the arterial loops of the glomerulus, is at first^shallow. Eventu- 

 ally the walls of the capsule grow about and enclose the vascular knot, except at 

 the point where the arteries enter and emerge (Fig. 212, 4 and 5). Renal cor- 

 puscles are first fully formed in 28 to 30 mm. embryos. The new corpuscles are 

 formed peripherally from persisting nephrogenic tissue until the tenth day after 

 birth, hence in the adult the oldest corpuscles are those next the medulla. Recon- 

 structions of the various stages in the development of the uriniferous tubules are 

 shown in Fig. 215. 



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

 renal artery of the metanephros (Broman, 1906). 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. Bremer (Amer. Jour. Anat., vol. 18, 1915) 

 derives the renal arteries not from the mesonephric vessels but from a periaortic plexus of 

 multiple aortic origin. s 



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 

 ("horse-shoe kidney"). Double or triple ureters and cleft ureters are sometimes present. 



DIFFERENTIATION OF CXOACA, BLADDER, URETHRA AND UROGENITAL 



SINUS 



In embryos of 1.4 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. 216 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. 216 B). 



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

 buds of the mesonephric ducts (Fig. 216 C, D). Next, the saddle-like partition 

 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 IS 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 



