DERIVATION OF THE MESOBLAST. 343 



Secondly, the body-cavity when it appears in the mesoblast 

 plates, docs not arise as a single cavity, but as a pair of cavities, 

 one for each plate of mesoblast, and these cavities remain 

 permanently distinct in some parts of the body, and nowhere 

 unite till a comparatively late period. Thirdly, the primitive 

 body-cavity of the embryo is not confined to the region in 

 which a body-cavity exists in the adult, but extends to the 

 summit of the muscle- plates, at first separating parts which 

 become completely fused in the adult to form the great lateral 

 muscles of the body. It is difficult to understand how the body- 

 cavity could have such an extension as this, on the supposition 

 that it represents a primitive split in the mesoblast between 

 the wall of the gut and the body-wall; but its extension to this 

 part is quite intelligible, on the supposition that it represents 

 the cavities of two diverticula of the alimentary tract, from 

 whose muscular walls the voluntary muscular system has been 

 derived. Lastly, I would point out that the derivation of part 

 of the muscular system from what appears as the splanchno- 

 pleure is quite intelligible on the assumed hypothesis, but, as 

 far as I see, on no other. 



Such are the main features presented by the mesoblast in 

 Elasmobranchs, 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), as a consequence of this, of the body- 

 cavity never communicating with the alimentary canal. These 

 points, in view of our knowledge of embryological modifications, 

 cannot be regarded as great difficulties to my view. We have 

 many examples of organs, which, though in most cases arising 

 as involutions, yet appear in other cases as solid ingrowths. 

 Such examples are afforded by the optic vesicle, auditory 

 vesicle, and probably also by the central nervous system, of 

 Osseous Fish. In most Vertebrates these organs are formed as 

 hollow involutions from the exterior; in Osseous Fish, however, 

 as solid involutions, in which a cavity secondarily appears. 



The segmental duct of Elasmobranchs or the Wolffian duct 

 (segmental duct) of Birds are cases of a similar kind, being 

 organs which must originally have been formed as hollow 

 involutions, but which now arise as solid bodies. 



