COMPARISON OF THE GERMINAL LAYERS. 



297 



Such are the main features, presented by the mesoblast 

 in Elasmobranchii, which favour the view of its having originally 

 formed the walls of the alimentary diverticula. Against this 

 view of its nature are the facts (i) of the mesoblast plates being 

 at first solid, and (2) of the 

 body cavity as a consequence 

 of this never communicating 

 with the alimentary canal. 

 These points, in view of our 

 knowledge of embryological 

 modifications, cannot be re- 

 garded as great difficulties 

 in my hypothesis. We have 

 many examples of organs, 

 which, though in most cases 

 arising as involutions, yet 

 appear in other cases as 

 solid ingrowths. Such ex- 

 amples are afforded by the 



optic vesicle, auditory vesicle, 

 and probably also by the 



FIG. 186. HORIZONTAL SECTION 

 THROUGH THE TRUNK OF AN EMBRYO OF 

 SCYLLIUM CONSIDERABLY YOUNGER THAN 



28 F. 



The section is taken at the level of the 



notochord, and shews the separation of the 



cells to form the vertebral bodies from the 

 muscle-plates. 



ill. notochord ; ep. epiblast ; Vr. rudiment 

 of vertebral body ; mp. muscle-plate ; nip' . 

 portion of muscle-plate already differentiated 



central nervous system of into longitudinal muscles. 



Osseous Fishes. In most Vertebrates these organs are formed 



as hollow involutions from the exterior ; in Osseous Fishes, 



however, as solid involutions, in which a cavity is secondarily 



established. 



There are strong grounds for thinking that in all Vertebrates 

 the mesoblast plates on each side of the notochord originate 

 independently, much as in Elasmobranchii, and that the noto- 

 chord is derived from the axial hypoblast ; but there are some 

 difficulties in the application of this general statement to all 

 cases. In Amphibia, Ganoids, and Petromyzon, where the 

 dorsal hypoblast is formed by a process of invagination as 

 in Amphioxus, the dorsal mesoblast also owes its origin to this 

 invagination, in that the indifferent invaginated layer becomes 

 divided into hypoblast and mesoblast. Amongst these forms 

 the mesoblast sheet, when separated from the hypoblast, is 

 certainly not continuous across the middle line in Petromyzon 

 (Calberla) and the Newt (Scott and Osborn), and doubtfully so 



