Excrelory and Reproductive Systems 81 



dominal cavity and in close relation to the developing kidneys. They 

 may later shift to a more posterior position. 



Urinogenital Ducts 



Outlets for the gonads and kidneys are provided by ducts which 

 lead into the cloaca. Two pairs of such ducts develop in nearly all 

 vertebrate embryos, the exceptions being some fishes. In conjunction 

 with the development of the pronephric tubules, a pair of ducts are 

 formed, each connecting the pronephric tubules of one side of the body 

 with the cloaca. These pronephric ducts extend back along the dorsal 

 body-wall. The mesonephric tubules, developing later and just behind 

 the pronephric tubules, acquire connection with the adjacent regions of 

 the pronephric ducts (Fig. 77). With the eventual degeneration of the 

 pronephric tubules, the corresponding part of each pronephric duct 

 degenerates, or at least ceases to be concerned with the kidneys. The 

 remaining and decidedly major part of the pronephric duct thereby be- 

 comes a mesonephric duct, otherwise commonly known as the 

 "Wolffian duct" (Fig. 78, w). Meanwhile, a second duct, known as 

 the "Miillerian duct," is formed lying closely lateral and parallel 

 to each Wolffian duct and opening into the cloaca (Fig. 78, m) . These 

 embryonic Mullerian ducts become the functional oviducts of the 

 female but are represented only by vestiges in adult males. The 

 Wolffian ducts serve as renal (urinary) ducts in both sexes, and in 

 males are urinogenital, providing outlet for both kidney and testis. 



Pronephros, Mesonephros, Metainephros 



The pronephros is functional in the free-living larvae of some 

 fishes and of amphibians, and in the adults of a very few bony fishes. 

 The mesonephros is certainly functional in the adults of the great 

 majority of fishes and amphibians, but in many cases there is doubt 

 as to the nature of the posterior part of the kidney in these animals, by 

 reason of the fact that the kidney extends into the extreme posterior 

 region of the abdominal cavity, thereby intruding into metanephric 

 territory. The distinction between mesonephros and metanephros prob- 

 ably must be based more on manner of development than on position. 

 The mesonephric tubules, formed from mesoderm just ventral to the 

 muscle-forming myotomes (Fig. 79), acquire direct connection with the 

 adjacent region of the prospective Wolffian duct ("primitive duct") 

 (Figs. 77, 80, 81). Development of a typical metanephros is initiated 

 by outgrowth of a forward-growing tubular branch from the cloacal 

 end of each Wolffian duct. This pair of branches, extending forward in 

 the dorsal body-wall, become the ureters of the prospective metanephric 

 kidneys (Figs. 77, 82). The tubules of the metanephros are derived 



