GERMIx^AL LAYERS OF THE EMBRYO. 
269 
before the origin to the vaso-peritoneal outgrowths (figs. 2 
and 13). 
The first of these processes suggests the view that the 
whole of the mesoblast primitively arose by a process of 
histogenic differentiation from the walls of the archenteron. 
This view, which was originally put forward by myself 
(No. 4), appears at first sight very improbable, but it re- 
ceives great support from the enormous development of the 
hypoblastic muscular system (Hertwigs, No. 13) in many 
Actinozoa. Lankester (No. 17), on the other hand, has urged 
that the mode of origin of the mesoblast in the Echinoder- 
mata is more primitive ; and that the amoeboid cells which 
here give rise to the muscular and connective tissue repre- 
sent 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 arise from the hypohlast, 
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. 
Reserving for the moment the question as to what con- 
clusions can be deduced from the above facts as to the origin 
of the mesoblast, it is important to determine how far the 
facts of embryology warrant us in supposing that in the 
whole of the triploblastic forms the body cavity originated 
from the alimentary diverticula. There can be but little 
doubt that the mode of origin of the mesoblast in the Verte- 
brata, as two solid plates split off from the hypoblast in 
which a cavity is secondarily developed, is an abbreviation of 
Fig. 1G. — Two Sections of a young TUlasmohranch Embryo, to show the meso- 
blast spilt off as two lateral rnasses from the hypoblast, mg. medullary 
groove; ^7?. epiblast ; »/ 2 . mesoblast; hypoblast. 
