DEVELOPMENT OF THE URINARY ORGANS. 



1939 



Allantoic 

 duct 



Tail 



1 ..._ 2\ 



jUreter \ \ Notoch 

 Rectum Wolffian duct Renal pelvis 



>rd 



Reconstruction of cloacal region of human embryo of thirty-three 

 days (11.5 mm. length) ; cloaca now incompletely separated into uro- 

 genital and intestinal segments. X 25. (Drawn from Keibel model.) 



the end-segment of the gut dilated into an elongated chamber, the cloaca, from the 

 upper end of which the allantois passes forward and on the sides of which open the 

 Wolffian ducts. The ventral wall of this space is thin, and consists of the opposed 

 outer and inner germ-layers alone, no mesoblast intervening. This ecto-entoblastic 

 septum is the cloacal mem- 

 brane. During the fourth week FIG. 1647. 



the Subdivision of the cloaca Venacava Aorta 



into a ventral and a dorsal 

 compartment begins by the for- 

 mation of a frontal fold that Belly-stalk-, 

 projects downward from the 

 angle between the gut and the 

 allantois. Subsequently this 

 partition is supplemented by 

 two lateral folds that appear on 

 the side walls of the cloaca and 

 are continuous above with the 

 frontal fold (Fig. 1646). By 

 the union of these three plicae, 

 above and from the sides, a 

 septum is formed that gradu- 

 ally grows caudally and sub- 

 divides the cloaca into a ventral 

 allantoic and a dorsal intestinal chamber. This partition, however, for a time is incom- 

 plete below, communication between the two spaces being thus maintained. 



During these changes the short canals common to the Wolffian ducts and the 

 primitive ureters are drawn into the ventral chamber, the four tubes thereafter open- 

 ing independently, but in close proximity, on the posterior wall of the ventral cloaco- 

 allantoic space. This undergoes further differentiation into an upper (vesical) and a 

 lower (genital) segment, the latter gradually narrowing into a tubular space, closed 

 below by the fore part of the cloacal membrane, which becomes the uro-genital 

 sinus and, after rupture of the membranous floor, communicates with the exterior. 

 For a time the orifices of the Wolffian ducts and the ureters are closely grouped, 



those of the former, how- 



FIG. 1648 ever, lying nearer the 



mid-line and slightly 

 higher than the more 

 widely separated ureteral 

 openings. 



During the second 

 month an important 

 modification of these 

 relations occurs, associ- 

 ated with elongation and 

 expansion of the upper 

 part of the vesical seg- 

 ment, by which the ure- 

 ters are drawn upward 

 and the Wolffian ducts 

 downward. The inter- 



Woiffian duct Ureter vening tract corresponds 



to the lower segment of 

 a spindle-shaped sac that 

 extends upward and is 





Tail 



Rectum 



Notochord 



Reconstruction of cloacal region of human embryo of thirty-seven days 

 (14 mm. length); ureter now opens independently into uro-genital sinus, 

 which above contributes lower segment of bladder and below is now almost 

 separated from gut-tube. X 17. (Drawn from Keibel model ) 



continued towards the 

 umbilicus by the allantois. The upper part of this sac, which is the dilated allantois, 

 forms the body and summit of the bladder and the urachus ; the lower part, into 

 which the ureters open (Fig. 1649) and which is derived from both allantois and 

 cloaca, differentiates into the vesical trigone and the urethra as far as the openings of 



