GERMINAL LAYERS OF THE EMBRYO. 
261 
tective layer, while the hypoblast was the secretory layer ; 
both layers giving rise to muscular elements. It must not, 
however, be supposed that in the early diploblastic ances- 
tors there Avas a complete differentiation of function, but 
there is reason to think that both the primary layers 
retained an indefinite capacity for developing into any 
form of tissue.^ The fact of the triploblastic condition 
being later than the diploblastic proves in a conclusive way 
that the mesoblast is a derivate of one or both the primary 
layers. In the Coelenterata Ave can study the actual origin 
from the tAvo primary layers of various forms of tissue Avhich 
in the higher types become the mesoblast.^ This fact, as 
Avell as general a priori considerations, conclusively prove 
that the mesohlast did not at first originate as a mass of 
independent cells between the txoo primarxj layers j hut that in 
the first instance it arose as histological differentiations of the 
two layers, and that its condition in the embryo as an inde- 
pendent layer of undifferentiated cells is a secondary condi- 
tion, brought about by the general tendency towards a simpli- 
fication of development, and a retardation of histological 
differentiation.^ 
In addition to the probably degraded Dicyemidse and Or- 
thonectidse, the Coelenterata are the only group in Avhich a 
completely differentiated mesoblast is not ahvays present. 
In other Avords, the Coelenterata are the only group in Avhich 
there is not found in the embryo an undifferentiated mass 
of cells from which the majority of the organs situated 
betAA^en the epidermis and the alimentary epithelium are 
dveloped. 
^ The Hertwigs (No. 13) have shoAVii that nervous structures are deve- 
loped in the hypoblast in the Actinozoa. 
“ There is considerable confusion in the use of the names for tlie em- 
bryonic 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 nomen- 
clature. In some of the Coelenterata it is difficult to decide in certain 
instances {e.g. Syinpodium) 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 arise as already histologically differentiated 
elements from one of the primary layers. The attempt to distinguish by^a 
special nomenclature the cpiblast and hypoblast after and before the separa- 
tion of the mesoblast, which has been made by Allen Thomson (No. 1), 
appears to me inconvenient in practice. 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, and 
only likely to introduce confusion into an already complicated nomenclature. 
^ The causes which give rise to a retardation of histological differentiation 
will be dealt with in a sequel to this paper treating of larval characters and 
larval forms. 
