120 DERIVATION OF THE MESOBLAST. 



is explained by him^as a secondary condition, a possibility wliich 

 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 primi- 

 tively 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'. 



From the somatic layer of the mesoblast there would no 

 doubt be formed the whole of the voluntary muscular system of 

 the bodv, the dermis, the subcutaneous connective tissue, and 

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

 thouo-h 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 

 discrepancy between theory and fact becomes very remark- 

 able. 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 the body, 

 including the first-formed muscles. Though remarkable, it is 

 nevertheless not inconceivable, that the skeleton might be 

 derived from the splanchoic mesoblast, but it is very difficult to 

 understand how there could be formed from it a part of the 

 voluntary muscular system of the body indistinguishably fused 

 with part of the muscular system derived from the somatopleure. 



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

 into a somatic and splanchnic layer as a secondary process {Gastrula u. Eifur- 

 chiuu) d. Thiere), but does not make it clear whether he regards this secondary 

 Bplitting as taking place along the old lines. It appears to me to be fauiy 

 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 

 alult 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 splanchuo- 

 pleure from the primitive somatopleure. 



