346 ORIGIN OF THE MESOBLAST. 



rise to almost the whole of the alimentary canal with its appen- 

 dages, except the liver. 



The origin of the Mesoblast. A diploblastic condition of 

 the organism preceded, as we have seen, the triploblastic. The 

 epiblast during the diploblastic condition was, as appears from 

 such forms as Hydra, especially the sensory and protective 

 layer, while the hypoblast was the secretory and assimilating 

 layer; both layers giving rise to muscular elements. It must 

 not, however, be supposed that in the early diploblastic ancestors 

 there was a complete differentiation of function, but there is 

 reason to think that both the primary layers retained an 

 indefinite capacity for developing into any form of tissue 1 . The 

 fact of the triploblastic condition being later than the diploblastic 

 proves in a conclusive way that the mesoblast is a derivative of 

 one or both the primary layers. In the Ccelenterata we can 

 study the actual origin from the two primary layers of various 

 forms of tissue which in the higher types are derived from the 

 mesoblast 2 . This fact, as well as general a priori considerations, 

 conclusively prove that the mesoblast did not at first 

 originate as a mass of independent cells between the 

 two primary layers, but that in the first instance it 

 gradually arose as differentiations of the two layers, 

 and that its condition in the embryo as an indepen- 

 dent layer of undifferentiated cells is a secondary 

 condition, brought about by the general tendency 



1 The Hertwigs (No. 270) have for instance shewn that nervous structures are 

 developed in the hypoblast in the Actinozoa and other Coelenterata. 



2 There is considerable confusion in the use of the names for the embryonic layers. 

 In some cases various tissues formed by differentiations of the primary layers have 

 been called mesoblast. Schultze, and more recently the Hertwigs, have pointed out 

 the inconvenience of this nomenclature. In the case of the Coelenterata it is difficult 

 to decide in certain instances (e.g. Sympodium) whether the cells which give rise to a 

 particular tissue of the adult are to be regarded as forming a mesoblast, i.e. a middle 

 undifferentiated layer of cells, or whether they arise as already histologically differ- 

 entiated elements from one of the primary layers. The attempt to distinguish by a 

 special nomenclature the epiblast and hypoblast after and before the separation of the 

 mesoblast, which has been made by Allen Thomson (No. 1), appears incapable of 

 being consistently applied, though it is convenient to distinguish a primary and a 

 secondary hypoblast. A proposal of the Hertwigs to adopt special names for the 

 outer and inner limiting membranes of the adult, and for the interposed mass of 

 organs, appears to me unnecessary. 



