UROGENITAL ORGANS 



371 



medulla is radially lined, since here are the straight portions of 

 Henle's loops and the collecting tubules. Cortex and medulla form 

 two series of interlocking pyramids, the result of the primitive lobu- 

 lation of the metanephros. 



The ureters, the history of which is given on p. 344, are free for 

 most of their course. In the early stages they open at the base of the 

 allantoic outgrowth (p. 346) into the same part of the cloaca as do the 

 reproductive ducts (fig. 394), this region being somewhat separated 

 from the rest as a urogenital sinus, the individuality of which becomes 



Fig. 395. — Model of pelvic region of human embryo 25 mm. long, after Keiiael. 

 (Compare with fig. 394.) a, anal opening; /, lateral ligament of uterus; m, MUllerian 

 duct; 0, ovary; />», primitive ureter (Wolffian duct); r, rectum; s, symphysis pubis; sg, 

 septum of genital protuberance; sug, urogenital sinus; u, ureter; ub, urinary bladder; 

 ur, recto-uterine excavation. 



more marked with increasing age (fig. 395, sug). In the mono- 

 tremes this condition is permanent, the ureters opening on papillae 

 opposite the base of the allantois, the proximal portion of which 

 expands to form the urinary bladder. In all other mammals the 

 openings of the ureters is included in the bladder and in these the 

 urine only reaches the urogenital sinus by the allantoic stalk, now 

 called the urethra, a name which is applied in the male to the whole 

 tube leading to the exterior, which can be described only after a con- 

 sideration of the reproductive organs. 



In the history of the reproductive organs the following parts are 



