THE BLADDER 369 



line. But, if the cells of the deeper strata are thrown off, it 

 becomes more difficult to determine their origin since they are 

 provided with tail-like processes in both the kidney and the 

 bladder. A marked predominance of these cells in the 

 deposit is more suggestive of pyelitis than of cystitis (Sahli). 



The large cells of the pavement epithelium of the vagina or 

 prepuce are frequently found in the urine. 



DEVELOPMENT OF THE BLADDER. The whole of the urinary 

 bladder is derived from the ventral portion of the cloaca. 



At the end of the third week of foetal life, the alimentary 

 canal consists of a short tube, closed at both ends but com- 

 municating freely with the yolk sac on its ventral surface 

 (Fig. i). A little caudal to this communication a short 

 blind diverticulum extends from the ventral aspect of the 

 gut into the body-stalk. This diverticulum is termed the 

 allantois, and it plays an important part in the development of 

 the bladder in some mammals. In man, however, it is very 

 small, and, as will be described, it takes no part in the forma- 

 tion of the bladder. 



On the cephalad aspect of the angle between the allantois 

 and the hind-gut, a transverse mesodermal septum grows tail- 

 wards and subdivides the cloaca into a ventral, urinary, and a 

 dorsal, gut, segment (Fig. 129). The Wolffian ducts (p. 380) 

 establish connexions with the lateral aspects of the cloaca, 

 and when the latter becomes subdivided, they maintain their 

 connexion with its ventral portion. Before the subdivision 

 of the cloaca is completely effected, the ureter arises as an 

 out-growth from the caudal extremity of the Wolffian duct and 

 grows headwards. Later, owing to a difference in the relative 

 rates of growth, the ureters come to open into the urinary 

 segment of the cloaca independently of the Wolffian ducts. 



It will been seen, therefore, that the bladder arises from the 

 ventral portion of the hind-gut, and that it is cut off from a 

 portion of the gut-tract, which, although very small originally, 

 is destined to form the whole of the large intestine. It is not 

 surprising, therefore, to find that the nerve-supply of the 

 24 



