310 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
One or another of these typical parts may be lacking in certain 
groups. Thus in the amniotes the nephrostomes are never formed, 
though ‘they occur in most ichthyopsida. In the pronephros the 
Malpighian corpuscle is rudimentary or lacking at all stages while 
there is no differentiation of convoluted tubules and Henle’s loop. 
The function of the various parts of the nephridial tubule is in 
outline as follows: Theoretically it would appear that in the primitive 
condition the nitrogenous waste, which is elaborated in the liver, 
collected in the ccelom and, together with the coelomic fluid, was 
passed outward through the nephrostomes and the tubules, which 
acted merely as ducts. Higher in the scale the parts become more 
differentiated and specialized. The renal corpuscles form a filtering 
apparatus by which water is passed from the blood-vessels of the glom- 
erulus into the tubules near their beginning, and this serves to carry 
out the urea, uric acid, etc., secreted by the glandular portions of the 
walls of the tubules (convoluted tubules, ascending limb of Henle’s 
loop). 
In development there may be three successive series of nephridial 
structures, the higher number occurring only in the amniotes. These 
are known as the pronephros (head kidney), mesonephros (Wolffian 
body), and the metanephros (permanent kidney of the amniotes). 
All three dre closely related in development and structure but are 
distinguished by differences in origin and in the final details. Three 
views are held as to their relations one to another. According to one 
they are parts of an originally continuous excretory organ (holone- 
phros) which extended the length of the body cavity. This has be- 
come broken up into the separate parts which differ merely in time 
of development and function, with minor modifications in details. A 
second view is that they are three separate organs, while a third regards 
them as superimposed structures which occasionally overlap (birds, 
gymnophiona) and thus are not, strictly speaking, homologous but 
rather homodynamous. The first view has the most in its support, 
but for convenience the three structures are kept distinct here. All 
arise from the mesomeric somites or from the Wolffian ridge which 
appears on either side of the median line where the mesomeres separate 
from the rest of the wall of the body cavity, the mesomeric cells furnish- 
ing the nephrogenous tissue from which the definitive organs develop. 
Pronephros.—The pronephros is the first to appear in develop- 
ment. As will be recalled (p. 14) the mesomere, like the epimere, 
