THE UROGEXITAL SYSTEM 



607 



the ventro-lateral walls of the myotomes, which normally give rise to 

 mesenchyme but which here evaginate in the pronephric region over 

 both dorsal and lateral surfaces of the head-kidney to meet with folds 

 coming up from the somatic layer of the lateral plate. This forms a 

 capsule of connective tissue which encloses not only the pronephros 

 proper, but also the pronephric sinus of the posterior cardinal vein. 



The pronephros is largest when the tadpole is about twelve milli- 

 meters long. It remains stationary until the twenty millimeter stage, 

 when it begins to degenerate. Degeneration is not quite complete at 

 metamorphosis. Various blind outgrowths of the three original tubules 



Fig. 350. 



Diagrams to show the development of the three kidneys and their ducts and 

 their relation to the male gonad. A, early stage rhovring the pronephros a, de- 

 veloping from the anterior end of the mesomere c and the pronephric duct b, which 

 has not yet reached the cloaca e. B, next stage illustrating the degeneration of 

 the pronephros at /, the development of the mesonephros h, from the middle portion 

 of the mesomere, the junction of the pronephric duct, now the mesonephric duct 

 O with the cloaca and the beginning of the metanephric evagination t from the 

 mesonephric duct. C, later stage, showing connection between certain tubules of 

 the mesonephros and the testis j by means of tubules, the vasa efferentia, p, 

 which grow out from the mesonephros ; and the penetration of the metanephric 

 evagination into the posterior end of the mesomere where it is subdividing to 

 form the collecting apparatus I, which becomes associated with the secretory 

 metanephric tubules m, developed in the mesomere. />, final stage, in which tjie 

 mesonephros has disappeared except for the remnant y, which connects with the 

 testis j by means of the vasa efferentia p; the tnesonephric duct g persists as the 

 vas deferens ; the two parts of the metanephros shown in C have united to form 

 a single organ r. a, pronephros ; 6, pronephric duct ; c, mesomere or nephrotome ; 

 d, intestine ; e, cloaca ; /, degenerating pronephros ; g, mesonephric or Wolffian 

 duct ; h, mesonephros or Wolffian body ; t, metanephric evagination from the 

 Wolffian duct in B, ureter in C and D; j, testis; fc, coiled portion of the vas 

 deferens forming part of the epididymis ; I, collecting part of the metanephros 

 derived from the Wolffian duct.; ra, excretory tubules of the metanephros derived 

 from the mesomere ; , nephrostome ; o, renal corpuscle or Malpighian body ; p, 

 vasa efferentia ; q, remnant of the mesonephros, forming part of the epididymis ; r, 

 metanephros. (From Hyman's "A Laboratory Manual for Comparative Vertebrate 

 Anatomy," by permission of The Chicago University Press.) 



