THE UROGENITAL SYSTEM 405 



in their eggs and which consequently have a prolonged larval period. 

 Usually pronephric tubules are not connected with glomeruli. The 

 associated glomeruli, instead of connecting directly with the tubules 

 through a Bowman's capsule, project into the body cavity in the neigh- 

 borhood of the nephrostomes. In this way wastes excreted into the body 

 cavity find their way indirectly to the tubules of the pronephros and 

 thus to the cloaca. In some cyclostomes and bony fishes, however, the 

 pronephros includes a segregated portion of the body cavity. This 

 becomes a pronephric chamber into which an inner glomerulus projects. 



It has been suggested that the glomeruli of the pronephros originally 

 belonged to the gills and not to the urinary system, and only with a reduc- 

 tion in the number of gills came to serve the pronephros. Such a sugges- 

 tion is quite consistent with the fact that the gills of fishes supplement the 

 kidneys as excretory organs. 



Mesonephros. The fact that the mesonephric tubules arise later 

 than the pronephric tubules during ontogenesis by no means proves that 

 they are phylogenetically younger, since the difference in time of appear- 

 ance in the embryo may be only an instance of the law of antero-posterior 

 differentiation in accordance with which anterior structures in the embryo 

 develop earlier than posterior ones. The chief reason for thinking that 

 the pronephros is phylogenetically older than the mesonephros is the fact 

 that the pronephros is more conspicuous in cyclostomes than in fishes and 

 amphibia. 



The mesonephros is the functional kidney of anamnia. Its tubules, 

 like those of the pronephros, are mesodermal and derived from the inter- 

 mediate cell mass, the nephrotomy They utilize the primitive (pro- 

 nephric) duct as an outlet. From their structure, development, 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 

 coelomoducts in each metamere, as in some annelids, and that in acrania 

 coelomoducts have degenerated while in the gill region nephridia have 

 persisted. 



It may be reasonably assumed that, like the pronephric tubules, those 

 of the mesonephros were metameric in origin. Indeed, they are metameric 

 in the embryos of elasmobranchs. In amniotes, however, the mesonephric 

 tubules branch repeatedly and lose their primary metamerism. In the 

 posterior portion of the mesonephros, the intermediate cell mass, nephro- 

 tome or mesomere, does not divide into segments, but forms its tubules 

 from an unsegmented mass. The mesonephric tubules differ from pro- 



