ORIGIN OF THE GERMINAL LAYERS. 287 



The Hertvvigs have recently attempted (No. 271) to distinguish 

 two types of differentiation of the mesoblast, viz. (1) a direct differen- 

 tiation 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 po.«sible that these cells may phylogenetically have origi- 

 nated from epithelial cells provided with contractile pi'ocesses 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 Dicyemidse and Ortho- 

 nectidge, the Coelenterata are the only group in which a true mesoblast 

 is not always present. In other words, the Coelenterata are the only k' 

 group in which there is not found in the embryo an undifferentiated 

 group of cells from which the majority of the organs situated be- 

 tween the epidermis and the alimentary epithelium are developed. 



The organs invariably derived, in the triploblastic forms, from 

 the mesoblast, are the vascular and lymphatic systems, the muscular 

 system, and the greater part of the connective tissue and the ex- 

 cretory and generative (?) systems. On the other hand, the nervous 

 systems (with a fevv possible exceptions) and organs of sense, the 

 epithelium of most glands, and a few exceptional connective-tissue 

 organs, as for example the notochord, are developed from the two 

 primary layers. 



The fact of the first named set of organs being invariably derived 

 from the mesoblast points to the establishment of the two following 

 propositions: — (1) That with the differentiation of the meso- ^ 

 blast as a distinct layer by the process already explained, 

 the two primary layers lost for the most part the capacity 

 they primitively possessed of giving rise to muscular and 

 connective-tissue differentiations\ to the epithelium of the 

 excretory organs, and to generative cells. (2) That the 

 mesoblast throughout the triploblastic Metazoa, in so far 

 as these forms have sprung from a common triploblastic 

 ancestor, is an homologous structure. 



The second proposition follows from the first. The mesoblast 

 can only have ceased to be homologous throughout the triploblastica \ 

 by additions from the two primary layers, and the existeoce of such ; 

 additions is negatived by the first proposition. 



^ The connective- tissue test of the Tunicata, though derived from the epiblast, is 

 not really an example of siich a differentiation. 



