DEVELOPMENT OF THE URINARY ORGANS. 



T939 



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 eloacal viem- 



FiG. 1647. 



Vena cava .Aorta 



Allantoic 

 duct 



Bellv-stalk 



Rectum 



I'rt'U-r 

 Wolffiau duct 



Tail ^_^ 



Notochord 

 Renal pelvis 



Reconstruction of eloacal region of human embryo of thirty-tin ee 

 days (II. 5 mm. length) ; cloaca now incompletely separated into uro- 

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



brane. During the fourth week 

 the subdivision of the cloaca 

 into a ventral and a dorsal 

 compartment begins by the for- 

 mation of a frontal fold that 

 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 

 primitiv'e 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 eloacal membrane, which becomes the ziro-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 exer, 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- 

 vening tract corresponds 

 to the lower segment of 

 a spindle-shaped sac that 

 extends upward and is 

 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 



Bladder 



Allantoic 

 duct 



Rectum 



Notochord 



Wolffian duct 



Ureter 



Reconstruction of eloacal region of human embryo of thirtv-seven days 

 (14 mm. length); ureter now opens independently' into uro-genita! sinus 

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

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



