THE ORIGIN OF THE HUMAN BODY 281 



nephridia (metanephric tubules). A brief description of the 

 three nephridial systems of vertebrate embryos will serve to 

 further clarify their interrelationship. 



(1) The pronephric system: This consists of a collection of 

 tubules called the pronephros, and a pronephric duct leading 

 to the cloaca, or terminal portion of the alimentary canal. 

 The pronephros is a functional organ in the frog tadpole and 

 other larval amphibia. It is also found in a few teleosts, 

 where it is said to persist as a functional organ in the adult. 

 In other fishes, however, and in all higher forms the pro- 

 nephros atrophies and becomes reduced to a few rudiments.^ 



(2) The mesonephric system: This consists of a collection 

 of nephridial tubules called the mesonephros (Wolffian body). 

 The tubules of the mesonephros do not develop any duct of 

 their own, but utilize the posterior portion of the pronephric 

 duct, the said tubules becoming secondarily connected with 

 this duct in a region posterior to the pronephridia (tubules 

 of the pronephros). The pronephric tubules together with the 

 anterior portion of the pronephric duct then atrophy, while 

 the persisting posterior portion of this duct receives the name 

 of mesonephric or Wolffian duct. The duct in question still 

 terminates in the cloaca, and serves, in the male, the combined 

 function of a urinary and spermatic duct; but, in the female, 

 a special oviduct (the Miillerian duct) is superadded because 

 of the large size of the eggs to be transmitted, the Wolffian or 

 mesonephric duct subserving only the urinary function. The 

 mesonephros is functional in mammalian embryos, but 

 atrophies and disappears coincidently with the development 

 of the permanent kidney. The same is true of amniotic verte- 

 brates generally, except that in the case of reptiles the meso- 



^The objection may be raised that a purely embryonic organ like 

 the pronephros, which is functional in but few vertebrate adults and 

 which originates in vertebrate embryos only to undergo atrophy, can 

 have no other explanation than that of "recapitulation." The objec- 

 tion, however, fails to take into account the possibility of the organ 

 being serviceable to the embryo, in which it may be a provisory solu- 

 tion of the excretory problem and not a vestige of past ancestry. 



