358 EVOLUTION OF THE MESOBLAST. 



less continuous sheet, split off from parts of one or both the 

 primary layers. It originates in most cases from the hypoblast, 

 and although the considerations already urged preclude us from 

 laying very great stress on this mode of origin, yet the deriva- 

 tion of the mesoblast from the walls of archenteric outgrowths 

 suggests the view that the whole, or at any rate the greater part, 

 of the mesoblast primitively arose by a process of histogenic 

 differentiation from the walls of the archenteron or rather from 

 diverticula of these walls. This view, which was originally put 

 forward by myself (No. 260), appears at first sight very 

 improbable, but if the statement of the Hertwigs (No. 270), that 

 there is a large development of a hypoblastic muscular system 

 in the Actinozoa, is well founded, it cannot be rejected as 

 impossible. Lankester (No. 279), on the other hand, has urged 

 that the mode of origin of the mesoblast in the Echinodermata 

 is more primitive ; and that the amoeboid cells which here give 

 rise to the muscular and connective tissues represent cells which 

 originally arose from the whole inner surface of the epiblast. It 

 is, however, to be noted that even in the Echinodermata the 

 amoeboid cells actually arise from the Jtypoblast, and their mode 

 of origin may, therefore, be used to support the view that the 

 main part of the muscular system of higher types is derived 

 from the primitive hypoblast. 



The great changes which have taken place in the develop- 

 ment of the mesoblast would be more intelligible on this view 

 than on the view that the major part of the mesoblast primitively 

 originated from the epiblast. The presence of food-yolk is 

 much more frequent in the hypoblast than in the epiblast ; and 

 it is well known that a large number of the changes in early 

 development are caused by food-yolk. If, therefore, the meso- 

 blast has been derived from the hypoblast, many more changes 

 might be expected to have been introduced into its early 

 development than if it had been derived from the epiblast. At 

 the same time the hypoblastic origin of the mesoblast would 

 assist in explaining how it has come about that the development 

 of the nervous system is almost always much less modified than 

 that of the mesoblast, and that the nervous system is not, as 

 might, on the grounds of analogy, have been anticipated, as a 

 rule secondarily developed in the mesoblast. 



