778 THE EXCRETORY AND REPRODUCTIVE SYSTEMS 



At the 16- to 21 -somite stage, the pronephric kidney is well developed, but 

 not all the tubules are present. At the 21 -somite stage, pronephric tubules 

 are present from the eleventh to fifteenth somites. Anterior to this area, they 

 are degenerate and rudimentary. At the 35-somite stage (65 to 70 hrs. of 

 incubation or stage 18, Hamburger and Hamilton, '51 ), the pronephric kidney 

 as a whole is undergoing degeneration, although the pronephric duct (now 

 the mesonephric duct) remains and, at this time, joins the dorso-lateral area 

 of the cloaca. 



e. Mammal (Human) 



In the human embryo, the pronephric rudiments extend from the seventh 

 to the fourteenth somites (fig. 344A), although rudimentary conditions may 

 extend as far forward as segment 2 (Felix, '12). The pronephric kidney 

 appears in embryos of about 9 to 10 pairs of somites, and begins to degenerate 

 at a stage of 23 to 28 segments. As in the chick and the shark, the pronephric 

 duct arises from the fusion of the dorso-lateral ends of the rudimentary pro- 

 nephric tubules and grows caudalward to open into the ventro-lateral aspect 

 of the cloaca in embryos of 4.2 mm., greatest length (fig. 344A). (See 

 Felix, '12.) 



Although the human pronephros is vestigial, it is as well developed as 

 in any other mammalia. 



4. Development of the Mesonephric Kidney 



The mesonephric kidney develops in the region of the nephrotomic plate 

 posterior to the pronephric kidney (fig. 344A). Five features distinguish 

 the mesonephric kidney from the pronephric kidney: 



( 1 ) The primitive segmentation manifest in the origin of the pronephric 

 kidney tubules is lacking generally in the mesonephric kidney, although 

 there is a tendency for the tubules to arise segmentally in the anterior 

 region. Also, a segmental origin of the tubules throughout the length 

 of the early mesonephros occurs in the embryo of the hagfish, Polis- 

 trotrema (Bdellostoma) stoiiti (Price, 1896), and a primitive seg- 

 mental condition is found in the early mesonephros of the shark and 

 frog embryos as indicated below. 



(2) The mesonephric tubules join the previously formed pronephric duct 

 and thus appropriate this duct. The pronephric duct then becomes 

 the mesonephric (Wolffian) duct. 



(3) The antero-posterior extent of the mesonephric kidney is much greater 

 than the pronephric kidney, the mesonephric kidney utilizing the 

 greater part of the nephrotomic plate. 



(4) An innovation, the collecting duct system, is introduced in the meso- 

 nephric kidney as a result of outgrowths from the mesonephric duct. 



