206 THE UROGENITAL SYSTEM 



tubule (intermediate piece of Felix). The primitive loop of Stoerck in- 

 cludes both the ascending and descending limbs of Henle's loop and a por- 

 tion 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. 214). 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. 212, 4 and 5). Renal corpuscles are first fully formed in 28 to 30 

 mm. embryos. The new corpuscles are formed peripherally from per- 

 sisting nephrogenic tissue until the tenth day after birth, hence in the 

 adult the oldest corpuscles are those next to the medulla. Reconstruc- 

 tions of the various stages in the development of the uriniferous tubules 

 are shown in Fig. 215. 



Renal Arteries. Bremer (1915) derives the renal arteries not from transformed meso- 

 nephric vessels, as did Broman (1906), but from a periaortic plexus of multiple aortic 

 origin. The mechanical selection of permanent channels explains the frequent variations 

 in the renal vessels. 



Anomalies. The kidneys may fail to ascend from their embryonic position in the 

 pelvis. Absence of one kidney is not infrequent. The kidneys sometimes fuse, either 

 completely into a disc-shaped mass, or partially by cortical union ('horse-shoe kidney'}; 

 in such cases the ducts usually are bilateral. Double or cleft ureters and pelves occur. 

 'Cystic kid ney' results when the uriniferous tubules fail to unite with the collecting tubules. 



DIFFERENTIATION OF THE CLOACA, 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. 2 1 6 A ) . Ventro-cephalad, the cloaca 

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

 laterally the mesonephric ducts and is prolonged caudally as the tail-gut 

 (Fig. 2i6B). 



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 uro genital 

 sinus. The division is complete in embryos of n 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 n mm., 

 according to Felix, the primitive urogenital sinus by elongation and con- 

 striction is differentiated into two regions: (i) 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 



