370 HISTORY OF THE HUMAN BODY 



the two tongues, the two sternums, the two sets of ribs and 

 the possibility of two mouths, but here we enter into a 

 greater complexity, for the history of the urinary organs in- 

 volves three kidneys, pronephros, mesonephros, and metanc- 

 phros, each with its associated parts, which represent as many 

 successive dynasties of organs that have replaced one another. 

 In cases like that of the two respiratory systems, where the 

 branchial system becomes replaced by the pulmonary, many of 

 the parts of the first become employed by the second, often 

 in quite a new capacity; but in the present case an added 

 element is introduced on the part of the neighboring repro- 

 ductive system, which not only employs at times portions of 

 one of the urinary systems, but retains them in its service long 

 after the system of which it formed a part has disappeared. 



The first, or pronephrotic, system, appears in the embryo 

 of all vertebrates; it functions during the larval life of some 

 fishes and amphibians (Fig. 106), and in a few teleosts per- 

 sists as a functional organ in the adult, but in other fishes and 

 in all higher forms it becomes reduced to a few rudiments. 

 It thus strongly suggests the assumption that it once formed 

 the functional kidney in some vertebrate ancestors, from which 

 it has been inherited. It consists of a few nephridial tubules, 

 strictly metameric in arrangement, that is, a pair for each 

 of several successive somites, situated very far anteriorly, 

 often involving the first of the trunk somites. The nephridia 

 of each side become associated together to form a single 

 kidney, the pronephros, and enter a common pr one phr otic 

 duct, laterally placed, and opening either directly to the ex- 

 terior in the vicinity of the cloacal opening or, more usually, 

 within the cloaca itself by means of a papilla which projects 

 from its dorsal wall. 



This duct is, for the most part, like the nephridia them- 

 selves, mesodermic in origin, although in some of the lower 

 forms the posterior portion arises from the ectoderm, giving 

 to the entire duct a double origin. This strange condition 

 may be in part accounted for if we consider that originally 

 there was a larger number of nephridia and that each opened 



