ORIGIN OF THE GERMINAL LAYERS. 



295 





and connective tissue systems as well as in the absence of a body cavity 

 resemble the Platyelininthes, may be cited in favour of this view, in that, 

 being closely related to the Chsetopoda, they are almost certainly descended 

 from ancestors with a true body cavity. The usual view of the primi- 

 tive character of the Platyelininthes, which has much to support it, is, 

 however, opposed to the idea that the body cavity has disappeared. 



If Kowalevsky ' is right in stat- 

 ing that he has found a form inter- 

 mediate between the Ccelenterata 

 and the Platyelininthes, 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. (1) a 

 more ancestral group (the Platyel- 

 minthes), in which there is no body 



cavity as distinct from the alimen- 

 tary, and (2) a group descended from 

 these, in which two of the aliinen- 



EIG. 214. SECTION THROUGH AN EM- 

 BBYO OF AGELENA LABYKINTHICA. 



The section is represented with the 



tary diverticula have become sepa- ventral plate upwards. In the ventral 

 rated from the alimentary tract to Pj ateig . see " a keel-like thickening, which 

 . J ... gives rise to the mam mass of the nieso- 

 lorm a body cavity (remaining tri- blast. 



ploblastica). However this may be, yjlf yo ik divided into large polygonal 



the above considerations are sufficient cells, in several of which are nuclei, 

 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 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 derivation 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 differ- 

 entiation from the walls of the archenterou or rather from diverticula 



1 Zoolocjisclier Anzeifler, No. 52, p. 140. This form has been named by Kowalevsky 

 Co:loplana Metxchnikoicii. 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. 



