378 HISTORY OF THE HUMAN BODY 



After birth a portion of this becomes shut within the body by 

 the closure of the umbilical connection, and as this portion is 

 in the form of an open bag leading out from the cloaca, it is 

 easily converted into a reservoir for urine, the greatest change 

 necessary being a slight shifting of the terminal portions of 

 the ureters. Only the lower portion is actually utilized for this 

 purpose, and the remainder atrophies into a ligament, which 

 extends from the apex of the bladder to the umbilicus. Ap- 

 proaching the cloaca the bladder becomes narrowed to a 

 small neck which is continued as a median duct or canal, the 

 urethra, and opens, in common with the genital ducts, into 

 the urogenital sinus. 



A structure called a urinary bladder is present in amphib- 

 ians. This is in the form of a collapsed bag leading out from 

 the ventral wall of the cloaca and is without direct connection 

 with the urinary system. This seems to represent morpholog- 

 ically an undeveloped allantois, and is thus really homologous 

 with the bladder of the Amniotes. Its function is not wholly 

 understood, as it never appears to contain liquid, but the occa- 

 sional presence within it of excretory salts suggests a sub- 

 ordinate use in connection with the urinary system. 



The second of the two associated systems to be considered 

 is that of reproduction (generation), and consists primarily 

 o the germ glands, in vertebrates a single pair, together with 

 some definite avenue of escape for the mature germ cells. To 

 these may be added secondarily external parts to insure the 

 union of the two sorts of germ cells. 



The germ glands, the essential organs of reproduction^ 

 develop as localized areas on the peritoneal wall of the ccelom, 

 and are primarily located dorsally, one on either side of the 

 vertebral column, in about the middle of the trunk region. 

 This similar origin, from a v layer which otherwise forms noth- 

 ing but investing membranes and suspensory ligaments, is 

 easily explained by the theory given above, which considers 

 the entire ccelom as the result of the fusion of a series of ex- 

 panded gonads, a theory perfectly in harmony with all the 

 related facts. The germ glands, primarily patches of germinal 



