ORIGIN OF THE GERMINAL LAYERS. 347 



towards a simplification of development, and a re- 

 tardation of histological differentiation 1 . 



The Hertwigs have recently attempted (No. 271) to distin- 

 guish two types of differentiation of the mesoblast, viz. (i) 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 origi- 

 nated 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 Dicyemida:: and Ortho- 

 nectidae, the Ccelenterata are the only group in which a true 

 mesoblast is not always present. In other words, the Ccelen- 

 terata 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. 



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 excretory and generative (?) systems. On the other 

 hand, the nervous systems (with a few possible exceptions) and 

 organs of sense, the epithelium of most glands, and a few 

 exceptional connective-tissue organs, as for example the noto- 

 chord, 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: (i) That with the differenti- 



1 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. 



