318 GILBERT C. BOURNE. 



According to MetschnikofF, in certain forms these cells are 

 marked out early in the embryo. 



In Alcyonaria cells in which the calcareous spicules forming 

 the skeleton are developed lie embedded in a gelatinous 

 matrix. 



In Actinaria and Madreporaria the supporting lamina is 

 fibrillar, and contains a few connective-tissue cells. Some- 

 times muscular fibres are embedded in it (Hertwigs, 15). 



It is obvious that in none of these cases (except the doubtful 

 case of the Ctenophora, as described by Metschnikofi") is there 

 anything like a true mesoblast, in the sense of a cellular layer 

 marked out early in the embryo. But there is a third layer 

 of tissue in the body, interposed between the ectoderm and 

 endoderm, which in some cases does and in others does not 

 contain cells, but the bulk of which in all cases is a gelatinous 

 matrix. This third layer assumes immense development in 

 some forms, e. g. the Discomedusae and Alcyonaria, so that it 

 is wholly misleading to call Coelenterates two-layered animals. 

 They are certainly three-layered — anyone can see that by 

 cutting a section across any one of them — but the question is. 

 Can they possibly be called triploblastic ? Can they be said to 

 possess a third germinal layer — a mesoblast? 



It is sometimes argued that the mesoblast is, after all, 

 nothing more than a layer of cells developed from one or both 

 of the two primary layers ; that the middle layer of the 

 Coelenterata contains cells in many instances ; that these cells 

 differ from the mesoblast cells of other forms only in the date 

 of their taking up their position in the third layer, the former 

 being separated ofi" from-the primary layers in the embryo, the 

 latter in the adult ; that this difi'erence in time is not essential ; 

 and that therefore the cell-containing middle layer in the 

 Alcyonaria, for example, has as much right to be called a 

 mesoblast as that of any other animal. 



I cannot but think that this style of argument leads to a 

 want of precision of ideas, and to a vagueness in the definition 

 of the thing signified. In a great number of forms the middle 

 layer contains no cellular element; it is a nearly structureless 



