ORIGIN OF THE GERMINAL LAYERS. 357 



Platyelminthes, which has much to support it, is, however, opposed to the 

 idea that the body cavity has disappeared. 



If Kowalevsky 1 is right in stating that he has found a form intermediate 

 between the Ccelenterata and the Platyelminthes, there will be strong grounds 

 for holding that the Platyelminthes are, like the Ccelenterata, forms the 

 ancestors of which were not provided with a body cavity. 



Perhaps the triploblastica are composed of two groups, viz. (i) a more 

 ancestral group (the Platyelminthes), in which there is no body cavity as dis- 



FIG. 214. SECTION THROUGH AN EMBRYO OK AGEI.ENA LABYRINTHICA. 

 The section is represented with the ventral plate upwards. In the ventral plate 

 is seen a keel-like thickening, which gives rise to the main mass of the mesoblast. 

 yk. yolk divided into large polygonal cells, in several of which are nuclei. 



tinct from the alimentary, and (2) a group descended from these, in which 

 two of the alimentary 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 shew how much there is 

 that is still obscure with reference 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 summary 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 



1 Zoologischer Anzeiger, No. 52, p. 140. This form has been named by Kowa- 

 levsky Cceloplana Metschnikowii. Kowalevsky's description appears, however, to be 

 quite compatible with the view that this form is a creeping Ctenophor, in no way 

 related to the Turbellarians. 



