286 ORIGIN OF THE MESO BLAST. 



but tlie limits of both these sections are so variable, sometimes 

 even in closely allied forms, that it is difficult to avoid the conclusion 

 that there is a border-land between the epiblast and hypoblast, 

 which appears by its development to belong in some forms to the 

 epiblast and in other forms to the hypoblast. If this is not the 

 case it is necessary to admit that there are instances in which a very 

 large portion of the alimentary canal is phylogenetically an epiblastic 

 structure. In some of the Isopods, for example, the stomodteum and 

 proctodseum give rise to almost the whole of the alimentar}'^ canal 

 with its appendages, 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 

 tissued 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 Coelenterata 

 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^ 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 independent layer of undifferentiated 

 cells is a secondary condition, brought about by the 

 general tendency towards a simplification of development, 

 and a retardation of histological differentiation^. 



^ The Hertwigs (No. 270) have for instance shewn that nervous structures are deve- 

 loped 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 incon- 

 venience 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 foimiug a mesoblast, i.e. a middle undifferentiated 

 layer of cells, or whether they arise as already histologically differentiated 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. i), appears incapable of being consistently applied, 

 though it is convenient to distinguish a primary and a secondary hypoblast. A jiro- 

 posal of the Hertwigs to adopt special names for the outer and inner limiting mem- 

 branes of the adult, and for the interposed mass of organs, appears to me unnecessary. 



* The causes which give rise to a retardation of histological differentiation will be 

 dealt with in the second part of this chapter which deals with larval characters and 

 larval forms. 



