THE URO-GENITAL SYSTEM 377 



origin in the two cases of such complicated structures as the 

 enal corpuscles. The primary location of the metane- 

 phros, posterior to that of the mesonephros, or at least to that 

 of its functional portion, leads to the idea that the nephridia 

 of this system were originally a part of the mesonephrotic 

 series, belonging to its more posterior somites, and that their 

 development from a structureless mass is a case of shortened 

 development, in which the primary segmental arrangement has 

 become lost. The necessity for the development of a new 

 ureter is easily seen in the employment of the older one (the 

 mesonephrotic or Wolffian duct), as a ductus [vas~[ deferens, 

 a point to be brought out later in connection with the re- 

 productive system. 



The external form of the metanephros varies consider?bly. 

 This in the Sauropsida is in accordance with the form of the 

 dorsal skeletal wall, to which it is closely applied. In struc- 

 ture it is usually distinctly divided into lobes that correspond 

 to the terminal branches of the ureter. This is characteristic 

 of the kidney of most mammals, and the compact form found 

 in Man is attained considerably after birth, and is met with in 

 only a few cases. 



As may be followed from the development, the ureters ter- 

 minate posteriorly in the mesonephrotic ducts and may be ex- 

 pected to share the common outlet into the cloaca. This is 

 actually the case in snakes, crocodiles and birds, which con- 

 sequently never perform urination as a distinct act, but in 

 other reptiles and in mammals there is found a terminal 

 resevoir, the urinary bladder, with which the ureters become 

 secondarily connected. This opens at first directly into the 

 cloaca, but its narrowed neck develops in the higher mammals 

 into a distinct canal, the urethra, which in the male comes into 

 direct association with the ductus deferens. The urinary blad- 

 der is no new formation, but is the remnant of the inner end 

 of the allantois, an extensive embryonal membrane, which 

 passes out of the body at the umbilicus and becomes in the 

 Sauropsida an external respiratory organ, and in mammals 

 furnishes the essential parts of the umbilical cord and placenta. 



