304 BULLETIN OF THE 



similar differeutiatiou of a part of the ccelom into a distinct excretory 

 chamber. The couditiou in tliis region differs from that of the meso- 

 nephros of this genus solely in tlie circumstance that the excretory cham- 

 ber is not broken up into metameric portions ; this process takes place 

 in the posterior region, and produces a typical mesonephros. 



It now remains for me to review the opinions of previous writers in 

 respect to the nature of the pronephi'os. The existence of a larval excre- 

 tory system different from and earlier than the mesonephros appears to 

 have been first suggested by Marcussen ('51) ; but this view received no 

 recognition until it had been reasserted by Wilh. Miiller ('75), who gave 

 to the pronephros a distinctive name, Vorniere. Semper ('75), on the 

 other hand, denied utterly the nephridial nature of the pronephros, and 

 regarded the glomus as equivalent to the suprarenals (Nebennieren) of 

 Plagiostomes. Furbringer ('78*) vigorously opposed this view, and main- 

 tained that the pronephros and its duct represent a primitive excretory 

 system which conspicuously differs from both mesonephros and meta- 

 nephros. According to Balfour's ('75) earlier view the segmental duct is 

 formed by the backward growth of a single anterior evagination, which 

 may be regarded as the representative of a mesonephric tubule. He 

 ('81) later interpreted the pronephros similarly to Fiirbringer, but was 

 still inclined to believe that each mesonephric tubule was " in a sort of 

 way serially homologous with the primitive pronephros." It is very 

 difficult for me to reconcile the latter opinion with his view that the 

 pronephros is a primitive excretory system derived from Plathelminthea, 

 while the mesonephros is a secondary (new) development which does not 

 appear until the trunk becomes segmented. Moreover, this view mani- 

 festly ignores the metamerism which is exhibited by the pronephros. It 

 appears to me therefore entirely unsatisfactory. 



Sedgwick ('81). first distinctly stated the conclusion that the pro- 

 nephros and mesonephros are differentiations of a single ancestral organ. 

 This view, which was adopted by Renson ('83), does not seem to have 

 been generally accepted, although several authors, by describing what 

 they denominate a transitional region, seem to me implicitly to assume 

 an intimate connection between the two glands. Mihalkovics ('85, pp. 

 65, 66) denied that they are wholly homologous, on the ground that the 

 .pronephric tubules are peritoneal evaginations, whereas those of the meso- 

 nephros are differentiated in the solid Wolffian blastema. Mihalkovics 

 does not explain his use of the term complete homology, and I have 

 been unable to satisfy myself in regard to the precise relations which 

 he supposed to exist between the two glands. 



