THE EXCRETORY ORGANS 339 



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. 362, 364) until it reaches the posterior end of the 

 metacoele, when it fuses with the hinder end of the digestive tract 

 (cloaca) or with the ectoderm in the vicinity of the anus. An open- 

 ing now breaks through into the cloaca, 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 metacoele. As the myotomes 

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

 so that eventually the ducts are just beneath the Uning of the defini- 

 tive body cavity. 



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

 the pronephric duct. They were long believed to be solely of mesothelial 

 character, arising by proliferation from the tube itself. Then it was noticed 

 that the backward-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 theo- 

 retical 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 nitrog- 

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

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

 meruli of the separate somites may run together, forming a glomus 

 (fig- 363). These glomeruli or the glomus of the pronephros do not 

 project into a Bowman's capsule, but lie immediately above the dorsal 

 wall of the coelom, between the mesentery and the nephrostomes 

 (fig. 364), pushing the epithelium before them. Later, as in the 

 caecilians, they and the nephrostomes may be enclosed in a cavity 



