338 DEVELOPMENT OF ELASMOBRANCH FISHES. 



subsequently becomes split into somatic and splanchnic strata. 

 This original fusion and subsequent splitting of the mesoblast 

 is explained by him as a secondary condition, a possibility 

 which cannot by any means be thrown on one side. It seems 

 therefore worth while examining how far the history of the 

 somatic and splanchnic layers of the mesoblast in Elasmobranchs 

 and other Vertebrates accords with the supposition that they 

 were primitively split off from the epiblast and the hypoblast 

 respectively. 



It is well to consider first of all what parts of the mesoblast 

 of the body might be expected to be derived from the somatic 

 and splanchnic layers on this view of their origin 1 . 



From the somatic layer of the mesoblast there would no 

 doubt be formed the whole of the voluntary muscular system of 

 the body, the dermis, the subcutaneous connective tissue, and 

 the connective tissue between the muscles. It is probable also, 

 though this point is less certain, that the skeleton would be 

 derived from the somatic layer. From the splanchnic layer 

 would be formed the connective tissue and muscular layers of 

 the alimentary tract, and possibly also the vascular system. 



Turning to the actual development of these parts, the dis- 

 crepancy between theory and fact becomes very remarkable. 

 From the somatic layer of the mesoblast, part of the voluntary 

 muscular system and the dermis is no doubt derived, but the 

 splanchnic layer supplies the material, not only for the muscular 

 wall of the digestive canal and the vascular system, but also for 

 the whole of the axial skeleton and a great part of the voluntary 

 muscular system of tJie body, including the first-formed muscles. 

 Though remarkable, it is nevertheless not inconceivable, that the 

 skeleton might be derived from the splanchnic mesoblast, but 



1 Professor Haeckel speaks of the splitting of the mesoblast in Vertebrates into 

 a somatic and splanchnic layer as a secondary process (Gastrula u. Eifiirchung d. 

 Thiere), but does not make it clear whether he regards this secondary splitting as 

 taking place along the old lines. It appears to me to be fairly certain that even if the 

 original unsplit condition of the mesoblast is to be regarded as a secondary condition, 

 yet that the splitting of this must take place along the old lines, otherwise a change in 

 the position of the body-cavity in the adult would have to be supposed an unlikely 

 change producing unnecessary complication. The succeeding argument is based on 

 .the assumption that the unsplit condition is a secondary condition, but that the split 

 which eventually appears in this occurs along the old lines, separating the primitive 

 splanchnopleure from the primitive somatopleure. 



