ORIGIN OF THE GERMINAL LAYERS. 



295 



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

 resemble the Platyelminthes, 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 Platyelminthes, which has much to support it, is, 

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

 If Kowalevsky' is i"ight in stat- 



ing that he has found a form inter- 

 mediate between the Ccelenterata 

 and the Platyelminthes, there will 

 be strong grounds for holding tliat 

 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 alimen- 

 tary diverticula have become sepa- 

 rated from the alimentary tract to 

 form a body cavity (remaining tri- 

 ploblastica). However this may be, 

 the above considerations are sufficient 



^^^^ 



^SA«. 



%. 



% 



Fig. 214. Section through an em- 

 bryo OF Agelena 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 meso- 

 blast. 



yk. yolk divided into large polygonal 

 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 refereuce 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 archentericy 

 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 archenteron or rather from diverticula i 



1 Zoolofjischer Anzeiijer, No. 52, p. 140. This form has been named by Kowalevsky 

 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. 



