HALL : MESOXEPILROS A\D MULLERIAX DUCT IN AMPHIBIA. 





primitive coelom. AVhile ventrally the coelom is uninterrupted, dorsally 

 it exists in the shape of separate pockets, so that the mesoderm has the 

 form of separate, hollow processes whose median walls are composed of 

 splanchnoderm, the lateral walls of somatoderm. These processes are 

 the somites. A diagrammatic cross-section of one is shown in Figure A. 

 They seem to he represented in their most primitive condition 

 in the elasmohranchs, where, according to Yan Wyhe ('89), the seg- 

 mentation extends ventrad even through the region of the germinal 

 epithelium.- Three regions may be more or less distinctly recognized 



^mer. 



ms'mer. 

 dt.fVf. 



Figures A-D. 



Four figures to illustrate the relation of the mesomer to the rest of the mesoderm. For 

 explanation see Introduction (page 32). The median plane of the body is at the 

 right in each case. 



in each somite. The most dorsal, to adopt the nomenclature of Van 

 Wyhe, is the epimer {e'mer., Fig. A), generally designated as the myotome 

 because the main trimk-muscles are derived from it. Its cavity is the 

 epicoelom. Passing ventrad, the next region is the mesomer (ins^mer.), 

 enclosing the mesocoelom. The mesomer has been designated by the 

 terms middle plate, intermediary cell-mass, "Urwirbelkommunikation," 

 etc. The sclerotome may be considered as arising from the upper por- 

 tion of its median wall, but the major part of both walls seems to 

 enter into the formation of the nephric fundaments. For this reason 



