THE ORGANS OF THE MIDDLE GERM-LAYER. 399 



sinus urogenitalis the urinary fluid and the male or female sexual 

 products. 



As far as regards the special conditions in Man, the allantois 

 remains in his case very small (fig. 132, 5 al) and possesses a lumen 

 in the region of the body-cavity only, whereas in the umbilical cord 

 and between the remaining fretal membranes only its connective- 

 tissue part, together with the blood-vessels, which shares largely in 

 the development of the placenta, grows further. In the second 

 month its hollow part, lying on the front wall of the abdomen, 

 becomes a spindle-shaped body (fig. 229 4 ). Its middle enlargement 

 becomes the urinary bladder (4), its upward prolongation, which 

 reaches to the navel, is called urachus ( 5 ), the other end (ug) is the 

 sinus urogenitalis. The urachus degenerates during embryonic life and 

 furnishes a connective-tissue cord, the ligamentum vesico-umbilicale 

 medium, which extends from the apex of the bladder (fig. 219 1M' ) 

 to the navel, and often in the first years after birth still contains an 

 epithelial cord, a remnant of the original epithelial canal. 



As is well known, the ureters (figs. 229 3 and 219 hi'} in the adult 

 open close together at the posterior surface of the urinary bladder 

 (229 4). In very young embryos this is not the case at first, for the 

 two ureters arise from the posterior part of the mesonephric duct, 

 and this opens into the sinus urogenitalis. But this condition is 

 soon altered. The ureter splits off from the mesonephric duct, 

 and comes to open independently into the posterior wall of the sinus 

 urogenitalis, from which it afterwards becomes gradually removed, 

 since its orifice, as it were, creeps higher up on the posterior wall of the 

 bladder. Like the change in the position of the sexual glands, we 

 must also conceive of this shifting as produced by processes of growth 

 in such a way that especially the tract between mesonephric duct 

 and ureter, which is at first small, increases in size, and thereby 

 produces the apparent upward migration of the opening of the 

 ureter. 



In the sixth week the cloaca in Man undergoes alterations which 

 are connected with the development of the external sexual organs. 

 The cloacal depression, which in earlier stages (fig. 230 A) appears 

 fissure-like, afterwards becomes (fig. 230 ) surrounded by a ring- 

 like fold, the genital ridge (gw), and there also arises in its anterior 

 portion a growth of connective tissue, which produces the externally 

 protruding genital eminence (gh). Along the lower surface of the 

 latter there is formed at the same time a groove (gr), which extends 

 downward to the cloaca, of which it is, as it were, the continuation. 



