THE REGION OF THE SPINAL CORD 393 



At present, then, Biickert'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 honio- 

 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 



