THE UROGENITAL SYSTEM 



295 



and relations, they are seen to be serially homologous with the pronephric 

 tubules. This is one of the facts which has led morphologists to postulate 

 an archinephros which consisted of a series of homologous tubules extend- 

 ing throughout the length of the trunk, and possibly, as in annelids, 

 throughout the entire length of the body. If the latter assumption is 

 made, it is also necessary to assume a pair of nephridia and a pair of 



NEPHROSTOME- 



.PRONEPHRIC 

 TUBULES 



A PRONEPHROS 



WOLFFIAN DUCT. 



B 



yMESONEPHR IC 

 TUBULES 



MESONEPHROS 



AFFERENT ARTERIOLE 

 EFFERENT ARTERIOLE 



GLOMERULUS 



NEPHROSTOME 



-WOLFFIAN DUCT- 



.......■.,1::^ .■j^^».i^^.^,.o.-a 



MrtiriMiiiniiin-r'irtr mfirnrtii iiTfafti -iiifiir-i'iiji'iltrliifTritfTiiiieigiMBaarMaitdaiiiit 



C MESONEPHROS D. METAMEPHROS 



Fig. 265. — Diagrams illustrating the relations of A, pro-, B-C, meso-, and D, meta- 

 nephridial tubules. The primary or pronephric tubules remove wastes from the 

 body-cavity, while the mesonephric tubules get their wastes directly from the blood; 

 many of them, however, retain the original connexion with the coelom. The meta- 

 nephric tubules lose all connexion with the coelom. (Redrawn after Corning.) 



coelomoducts in each metamere, as in some anneUds, and that in Acrania 

 coelomoducts have degenerated while in the gill region nephridia have 

 persisted. 



In the male the anterior portion of the mesonephros changes its func- 

 tion from that of an excretory organ to that of a reproductive organ. In 

 other words, the tubules of the anterior part of the mesonephros return 



