312 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 



grow outward until they are just beneath the ectoderm, when they 

 bend toward the posterior end of the body, the anterior tubules fusing 

 with those behind. From the junction a tube, the pronephric or 

 archinephric duct, gradually grows backward just beneath the 

 ectoderm (figs. 317, 318) until it reaches the posterior end of the meta- 

 ccele, when it fuses with the hinder end of the digestive tract (cloaca) 

 or with the ectoderm in the vicinity of the anus. An opening now 

 breaks through, thus putting the ccelom indirectly in communication 

 with the outer world. 



At first the pronephric duct lies closely below the ectoderm and is 

 almost equally near the lining of the metaccele. As the myotomes 

 grow downward they come to lie between the ducts and the ectoderm 

 so that eventually the ducts are just beneath the lining of the definitive 

 body cavity. 



There has been considerable dispute as to the origin of the cells which form the 

 pronephric duct. They were long thought to be solely of mesothelial character, 

 arising by proliferation from the tube itself. Then it was noticed that the back- 

 ward-growing tube fused at its tip with the ectoderm and it was thought that there 

 was an actual contribution of ectodermal cells at this point. This view received 

 considerable support from its agreement with certain theoretical views. The 

 matter is not yet decided. The writer is convinced, from the study of perfectly 

 preserved material in which cell boundaries are clearly shown, that in the sharks 

 (Acanthias] which were thought most strongly to support the view of ectodermal 

 contribution, that the whole duct is of mesothelial origin. 



In the teleosts the dorsal end of the nephrotome grows out to form the pro- 

 nephric tubule, to which both somatic and splanchnic walls thus contribute. In 

 the amphibia the nephrotome is not distinctly separated from the lateral plates 

 (hypomere) and the pronephric tubules are formed from the common area. 



The pronephros is functional for a time in the embryos of some 

 lower vertebrates; in other groups it is a rudimentary and transitory 

 structure, save for its participation in the oviducts and the ostium 

 tubae abdominale (see below). When functional it takes the nitro- 

 genous waste from the body cavity, while its filtering apparatus con- 

 sists either of separate glomeruli (one for each tubule) or the glomeruli 

 of the separate somites may run together, forming a glomus. These 

 glomeruli or the glomus of the pronephros do not project into a Bow- 

 man's capsule, but lie immediately above the dorsal wall of the ccelom, 

 between the mesentery and the nephrostomes (fig. 318), pushing the 

 epithelium before them. Later, as in the caecilians, they and the 

 nephrostomes may be enclosed in a cavity cut off from the ccelom, 



