374 HISTORY OF THE HUMAN BODY 



and that portion of the duct anterior to the connection with 

 the mesonephridia become atrophied, and the duct thus be- 

 comes the mesonephrotic, the " Wolffian duct " of an earlier 

 nomenclature. On account of this utilization of the prone- 

 phrotic duct by the mesonephrotic tubules it has been held by 

 some that both belong to one system and that the latter are 

 merely later appearing elements of the pronephrotic series, 

 but this is discredited by others on the ground of the differ- 

 ence in the time of functional activity of the two systems and 

 also on account of the several somites without nephridia of 

 either kind that intervene between the two. The structural 

 difference between the two types of nephridia, one with a 

 Bowman's capsule, the other without, may be also employed 

 as an argument in favor of the distinctness of the two systems, 

 but this argument is weakened by a consideration of the man- 

 ner of formation of this new part, and by the assumption of the 

 existence of almost every grade of transition between the two. 

 Thus in its more usual form a pronephrotic tubule is related 

 to the accompanying glomerulus much as in Fig. 108, A, but 

 in some cases the projection bearing the glomerulus becomes 

 partly enclosed in a recess of the ccelom, and the nephrostome 

 opens into this instead of into the main cavity (Fig. 108, 

 B). The development of cilia at the narrow passage which 

 leads into the recess, and the loss of them around the mar- 

 gin of the original nephrostome, would convert the entire 

 apparatus into a mesonephrotic tubule, in which the added por- 

 tion, including the new nephrostome and the Bowman's 

 capsule, is a contribution from the peritoneum. These dia- 

 grams seem absolutely persuasive, but unfortunately do not 

 correspond with the actual facts, since, although in special 

 cases the glomeruli of the pronephrotic system are related 

 much as in the second diagram, the Bowman's capsules of the 

 mesonephrotic tubules develop directly as evaginations from 

 their walls, and there is thus no indication of either the par- 

 ticipation of the ccelom or of the formation of a new nephro- 

 stome. 



The mesonephrotic system may be found in full functional 



