34 BULLETIN : MUSEUM OF COxMPAEATIVE ZOOLOGY. 



it has also been termed the nephrostome. The third, most ventral, por- 

 tion of the somite is the hypomer and gives rise to the germ cells (cLg.). 

 These originally arose from both layers,^ but now seem usually to 

 be confined to the splanchnoderm. Following on the hypomer comes 

 the unsegmented portion of the mesoderm, "lateral mesoderm," con- 

 sisting of the lateral plates enclosing the true body cavity (coel.). It 

 must be borne in mind that the three portions of the somite are distin- 

 guishable rather from their fate than by any early differences in shape 

 or histological structure. 



The various views held in regard to the origin of the mesonephric 

 units may be arranged in five categories as follows : the cells composing 

 the fundaments of the units are derived from (1) the Wolffian duct, (2) 

 the myotome, (3) the intermediary cell-mass, (-i) the peritoneal epithe- 

 lium, (5) mesoderm cells of unknown origin. 



The first view, that the Wolffian duct furnishes the fundaments of the 

 inesonepliric units, we may safely exclude, for there is but one group of 

 vertebrates (Teleosts) in which such an origin has been claimed of late 

 years (Felix, '97), and in this group it seems questionable not only whether 

 the organ described is really a mesonephros, but also whether the Wolf- 

 fian duct is truly homologous with that of other groups.^ 



In regard to the second, third, and fourth views, they are all recon- 

 cilable with the theory that the mesonephric tissue is always derived 

 from a portion of the mesothelium, the mesomer, which is homologous 

 throughout the vertebrate phylum. If the severance of the ventral 

 (lateral) portion of the mesoderm from the dorsal portion takes place 

 between mesomer and hypomer (Fig. B), the former may appear as an 

 integral part of the epimer, and on developing give rise to the impres- 

 sion that the sclerotome and mesonephros arise from the myotome. If 

 the division takes place between mesomer and epimer, the mesonephric 

 fundament will appear either as if derived from the intermediary cell- 

 mass (Fig. C), or, in case that structure has flattened out, as if from that 

 portion of the peritoneal epithelium immediately lateral (dorsal) to the 

 gonad (Fig. D). That the sclerotome never seems to arise directly from 

 the peritoneum is due to the fact that it is given off at too early a 

 stage. 



1 Rabl ('96) states that primitive germ cells are found in the somatoderm as 

 well as in the splanchnoderm in early stages of elasmobranch development, and I 

 find the same condition in Amphibia. 



2 Felix states that both somatoderm and splanchnoderm enter into the forma- 

 tion of the Wolffian duct in the trout. 



