244 FROM FISH TO PHILOSOPHER 



all discussions of the origin of the vertebrates, and to 

 leave that question as open as it was a century ago. 



That the primitive coelomic membrane possessed ex- 

 cretory powers is indicated by the role of this membrane 

 in the invertebrates, by vertebrate embryology, and by 

 the secretory properties of the pericardial and perivis- 

 ceral membranes in the fishes and reptiles, in which 

 these membranes are derived from the coelom {59, 60, 



61}. 



For the embryonic structure of the kidney in the 

 fishes see Bridge {49, p. 398 f}. An excellent smnmary 

 of the fxmction of the archinephric duct is given by 

 Romer {15, p. 389}, and the anatomy of the renal-portal 

 system is described by Fraser {51}. 



The reproductive cells continue to be discharged into 

 the coelomic cavity in the most primitive living fishes, 

 the cyclostomes, and escape from the body through a 

 pair of abdominal pores which open into the cloaca. Al- 

 though other routes of egress for the reproductive cells 

 have been established, abdominal pores persist in most 

 elasmobranch fishes and ia the lungfishes, PolypteruSy 

 the sturgeons, the paddlefish, the gar pike and bowfin 

 —all primitive fishes— but rarely in the Teleostei (bony 

 fishes). These pores may reflect the primitive mode of 

 reproduction, or they may reflect the primitive excretory 

 role of the coelomic cavity {49, p. 402}. 



Another primitive feature in the kidney is the per- 

 sistence in some sharks, the bowfin (Amia), and a few 

 Amphibia (the salamanders and a few frogs and toads) 

 of open coelomostomes draining into the renal tubules. 

 More generally, however, such coelomostomes drain into 

 the renal venous sinuses or the renal vein. In both in- 

 stances they serve, in the manner of lymphatics, to re- 

 turn coelomic fluid to the circulation. Though these per- 

 sistent coelomostomes may be conceived as a primitive 



