314 GILBERT C. BOUENE. 



median lamella^ and what is its origin in the Coelenterate 

 phylum ? 



This is not the place to enter into a discussion of the whole 

 subject of the origin and significance of the germinal layers, 

 which the first question introduces, but it may shortly be 

 stated, without going very far wrong, that by mesoblast is 

 meant a layer of undifferentiated cells, developed in the embryo 

 before the differentiation of other organs or tissues, from either 

 one or the other or both of the primary germ layers, the epi- 

 blast and hypoblast. By mesoderm, or its adjective meso- 

 dermic, are meant all such tissues in the adult as are clearly 

 derived from the mesoblast. This is not the sense in which 

 I should like to use the term, but a sense which has become 

 inevitable from the usage of other authors. 



To this idea of mesoblast recent theories on the origin of 

 metameric segmentation have added another highly important 

 signification, and one which is of especial importance to the 

 present question. In the majority of the higher Metazoa 

 (Triploblastica) the mesoblast is understood, in part, to denote 

 the limiting layer of the coelom. The Platyhelminthes offer a 

 difficulty to this conception in that they are not known to 

 possess a true coelom, and it is a question whether they ances- 

 trally possessed one, or whether they are the surviving repre- 

 sentatives of the triploblastic Metazoon in which the coelom 

 was not developed. From the analogy of the Discophora, and 

 from other considerations, I am inclined to think it probable 

 that future researches will prove that all the Triploblastica are 

 ancestrally Coelomata, the presence of mesoblast implying the 

 (ancestral) presence of a coelom.^ However this may be, in 



^ lu the embryo of Leptoplana, the cells which will form the mesoblast are 

 marked out very early, before the hypoblast and epiblast are defiuitely estab- 

 lished. But it is noticeable that the mesoblast cells are split off from the 

 four large cells which afterwards form the hypoblast, the epiblast having been 

 already marked out by four smaller cells, which eventually increase in number 

 and surround the mesoblast and hypoblast. I think that in this case the 

 mesoblast may fairly be said to have a hypoblastic origin. I can see no 

 objection to the view that this may be a very much abbreviated development, 

 derived from a type in which the mesoblast arose as (hypoblastic) outgrowths 



