THE REGION OF THE SPINAL CORD 393 
At present, then, Riickert’s view is the one most generally ac- 
cepted—the original annelid nephric organs are represented by the 
pronephric tubules and the pronephric duct, not by the mesonephric 
tubules, which are a later formation. This latter statement would 
hold good if the mesonephric tubules were found entirely in seg- 
ments posterior to those containing the pronephric tubules; such, 
however, is said not to be the case, for the two sets of organs are 
said to overlap in some cases; even when they exist in the same 
segments, the former are said always to be formed from a more 
dorsal part of the ccelom than the pronephros,-always to be a later 
formation, and never to give any indication of communicating with 
the exterior except by way of the pronephric duct. 
The recent observations of Brauer on the excretory organs of the 
Gymnophiona throw great doubt on the existence of mesonephric and 
pronephric tubules in the same segment. He criticizes the observa- 
tions on which such statements are based, and concludes that, as in 
Hypogeophis, the nephrotome which is cut off after the separation of 
the sclero-myotome gives origin to the pronephros in the more anterior 
regions, just as it gives origin to the mesonephros in the more 
posterior regions. In fact, the observations of van Wijhe and others 
do not in reality show that two excretory organs may be formed 
in one segment, the one mesonephric from the remains of the meso- 
mere and the other pronephric from the hypomere, but rather that 
in such cases there is only one organ—the pronephros—part of which 
is formed from the mesomere and part from the hypomere. Brauer 
goes further than this, and doubts the validity of any distinction 
between pronephros and mesonephros, on the ground of the former 
arising from a more ventral part of the proccelom than the latter ; 
for, as he says, it is only possible to speak of one part of the somite 
as being more ventral than another part when both parts are in the 
same segment; so that if pronephric and mesonephric organs are 
never in the same segment, we cannot say with certainty that the 
former arises more ventrally than the latter. 
These observations of Brauer strongly confirm Sedgwick’s original 
statement that the pronephric and mesonephric organs are homo- 
dynamous organs, in that they are both derived from the original 
serially situated nephric organs, the differences between them being 
of a subordinate nature and not sufficient to force us to believe that 
the mesonephros is an organ of quite different origin to the 
