394 THE ORIGIN OF VERTEBRATES 
pronephros. So, also, Price, from his investigations of the excretory 
organs of Bdellostoma, considers that in this animal both pro- 
nephros and mesonephros are derived from a common embryonic 
kidney, to which he gives the name holonephros. 
Brauer also is among those who conclude that the vertebrate 
excretory organs were derived from those of annelids; he thinks that 
the original ancestor possessed a series of similar organs over the 
whole pronephric and mesonephric regions, and that the anterior 
pronephric organs, which alone form the segmental duct, became 
modified for a larval existence—that their peculiarities were adaptive 
rather than ancestral. This last view seems to me very far-fetched, 
without any sufficient basis for its acceptance. According to the 
much more probable and reasonable view, the pronephros represents 
the oldest and original excretory organs, while the mesonephros 
is a later formation. Brauer’s evidence seems to me to signify that 
the pronephros, mesonephros, and metanephros are all serially homo- 
logous, and that the pronephros bears much the same relation to 
the mesonephros that the mesonephros does to the metanephros. 
The great distinction of the pronephros is that it, and it alone, 
forms the segmental duct. 
We may sum up the conclusions at which we have now arrived 
as follows :-—— 
1. The pronephric tubules and the pronephric duct are the oldest 
part of the excretory system, and are distinctly in evidence for a 
few segments only in the most anterior part of the trunk-region 
immediately following the branchial region. They differ also from 
the mesonephric tubules by not being so clearly segmental with the 
myotomes. 
2. The mesonephric tubules belong to segments posterior to those 
of the pronephros, are strictly segmental with the myotomes, and 
open into the pronephric duct. 
3. All observers are agreed that the two sets of excretory organs 
resemble each other in very many respects, as though they arose 
from the same series of primitive organs, and, according to Sedgwick 
and Brauer, no distinction of any importance does exist between 
the two sets of organs. Other observers, however, consider that the 
pronephric organs, in part at all events, arise from a part of the 
nephroccele more ventral than that which gives origin to the mesone- 
phric organs, and that this difference in position of origin, combined 
