PATHOLOGY OF THE BLADDER AND URETER. 381 



yolk sac and amniotic cavity through the medullary plate. The 

 rudimentary allantoic stalk is seen passing from yolk sac into 

 body stalk, but of a bilateral development of the allantois we 

 have no evidence in the human embryo (figs. 1, 4, 6-9). 



By a ventral unfolding of the blastodermic edge towards the 

 future navel, we get the primitive gut formed, and this accord- 

 ingly gives the area of the neur enteric canal a ventral position 

 -^-i.e., it is converted into part of the anterior boundary of the 

 primitive gut — the cloacal membrane. 



This is well displayed in Keibel's model of His' embryo E.B., 

 {fig. 1) 3 mm. long, and in the embryo HsJ., 4*2 mm. long. 



We see here the caudal division of the primitive gut, with the 

 "Wolffian duct opening into what will be its anterior bladder 

 division (HsJ, and Hsf.). There are two parts of this caudal 

 primitive gut — (1) the end-gut, which withers, and (2) the 

 portion between the future navel and end-gut known as the 

 entodermal cloaca. This is the gut, undivided as yet, from 

 which bladder and bowel are to arise. Lateral folds, and in all 

 probability a peritoneal vertical dip, divide off the primitive 

 gut into bladder in front and bowel behind, as in the guinea- 

 pig (figs. 6-9). As the Wolffian ducts open into the bladder 

 division, we understand how, after the ureter has budded off 

 from the Woffian duct (Hsf.), and the trigone of the bladder 

 developed at their junction, we get the ureter opening into 

 the bladder and the Wolffian ducts into the sinus lower down. 

 The after developtnetit of the Midlerian ducts in the female 

 carries the WolJ)ian openimjs further down to form the hymen. 

 The bladder is thus developed in the main from primitive gut, 

 only its apex being allantoic. 



A very important point as to the anterior boundary of the 

 entodermal cloaca now requires description. This is known as 

 the cloacal membrane, and, as was first pointed out by KoUiker, 

 and afterwards confirmed by others, it has at one time no 

 mesoblast, and is thus a part structurally weak. The upper 

 boundary of this membrane is the navel, and its lower the 

 junction of end-gut and entodermal cloaca. In the adult this 

 lower point is on the perineal body in front of the anus, as in 

 a rare malformation, viz., persistent end-gut in a female child ; 

 I found this part of the gut attached to skin in front of the 



