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 Jwlonejrfiros. 



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 

 nephrocoele more ventral than that which gives origin to the mesone- 

 phric organs, and that this difference in position of origin, combined 



