GERMINAL LAYERS OF THE EMBRYO. 271 
in the Platyelminthes, and become lost, and even in some 
cases replaced physiologically by alimentary diverticula. 
The usual view of the primitive character of the Platyel- 
minthes, which has much to support it, is, however, opposed 
to the idea that the body cavity has disappeared. 
If Kowalewsky^ is right in stating that he has found a 
form intermediate between the Coelenterata and the Platyel- 
minthes, there will be strong grounds for holding that the 
Platyelminthes are, like the Coelenterata, forms the ancestors 
of which were never provided with a body cavity. 
Perhaps the triploblastica are composed of two groups, viz. 
(1) a more ancestral group (the Platyelminthes), in which 
there is no body cavity as distinct from the alimentary, and 
(2) a group descended from these, in which two of the ali- 
mentary diverticula have become separated from the alimen- 
tary tract to form a body cavity (remaining triploblastica). 
However this may be, the above considerations are sufficient 
to show how much there is that is still obscure with refer- 
ence even to the body cavity. 
If embryology gives no certain sound as to the questions 
just raised with reference to the body cavity, still less is it 
to be hoped that the remaining questions with reference 
to the origin of the mesoblast can be satisfactorily answered. 
It is clear, in the first place, from an inspection of the sum- 
mary given above, that the process of development of the 
mesoblast is, in all the higher forms, very much abbreviated 
and modified. Not only is its differentiation relatively 
deferred, but it does not in most cases originate, as it must 
have done to start with, as a more or less continuous sheet, 
split off from one or both the primary layers. It originates 
in most cases from the hypoblast, and although the con- 
siderations already urged preclude us from laying very great 
stress on this mode of origin ; yet, as suggested above, it 
appears to me not impossible, judging from the analogy of the 
Actinozoa, that the muscular system of the triploblastica 
may have primitively mainly arisen from differentiations of 
the hypoblast of the alimentary diverticula, which seem to 
have given rise to the body cavity. 
The great changes which have taken place in the de- 
velopment of the mesoblast would be more intelligible on this 
view than on the view of the mesoblast having primitively 
largely originated from the epiblast. The presence of food- 
‘ ‘ Zoologischer Auzeiger,’ No. 52, p. 140. This form Las been named by 
Kovvalewsky Coleoplana Metschnikowii. Kowalewsky’s deseriptiou appears, 
however, to be quite com))atible willi the view that this form is a creeping 
Ctenophor, in no way related to the Turbellarians. 
