[125] 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 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. 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. [126] 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. The Hertwigs have recently attempted (No. 271) to distinguish two types of differentiation of the mesoblast, viz. (1) a direct differentiation from the primitive epithelial cells; (2) a differentiation from primitively indifferent cells budded off into the gelatinous matter between the two primary layers. It is quite possible that this distinction may be well founded, but no conclusive evidence of the occurrence of the second process has yet been adduced. The Ctenophora are the type upon which special stress is laid, but the early passage of amoeboid cells into the gelatinous tissue, which subsequently become muscular, is very probably an embryonic abbreviation; and it is quite possible that these cells may phylogenetically have originated from epithelial cells provided with contractile processes passing through the gelatinous tissue. The conversion of non-embryonic connective-tissue cells into muscle cells in the higher types has been described, but very much more evidence is required before it can be accepted as a common occurrence. In addition to the probably degraded Dicyemidæ and Orthonectidæ, the Coelenterata are the only group in which a true mesoblast is not always present. In other words, the Coelenterata are the only group in which there is not found in the embryo an undifferentiated group of cells from which the majority of the organs situated between the epidermis and the alimentary epithelium are developed.