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- 
coele, 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 
tube 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 cecilians, they and the 
nephrostomes may be enclosed in a cavity cut off from the ccelom, 
