356 



ORIGIN OF THE MESOBLAST. 



n.al 



Vertebrata to the ingrowth of the mesoblast from the lips of the 

 blastopore. 



It is, therefore, highly ^ m ^fJ j A 



probable that the paired in- 

 growths of the mesoblast from 

 the lips of the blastopore may 

 have been in the first instance 

 derived from a pair of arch- 

 enteric diverticula. This pro- 

 cess of formation of the meso- 

 blast is, as may be seen by 

 reference to the summary, 

 the most frequent, including 

 as it does the Chaetopoda, 



the Mollusca, the Arthro- FIG. 213. Two SECTIONS OF A YOUNG 



, o x ELASMOBRANCH EMBRYO, TO SHEW THE 



MESOBLAST SPLIT OFF AS TWO LATERAL 

 MASSES FROM THE HYPOBLAST. 



While there is no difficulty in 

 the view that the body cavity may 

 have originated from a pair of en- 

 teric diverticula in the case of the 

 forms where a body cavity is present, there is a considerable difficulty in 

 holding this view, for forms in which there is no body cavity distinct from 

 the alimentary diverticula. 



Of these types the Platyelminthes are the most striking. It is, no doubt, 

 possible that a body cavity may have existed in the Platyelminthes, and 

 become lost ; and the case of the Discophora, which in their muscular 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 Chaetopoda, they are almost certainly descended from ancestors 

 with a true body cavity. The usual view of the primitive character of the 



i- medullary groove ; cp. epiblast ; in. 

 mesoblast ; hy. hypoblast ; n.al. cells formed 

 around the nuclei of the yolk which have 

 entered the hypoblast. 



1 The wide occurrence of this process was first pointed out by Rabl. He holds, 

 however, a peculiar modification of the gastreea theory, for which I must refer the 

 reader to his paper (No. 284) ; according to this theory the mesoblast has sprung 

 from a zone of cells of the blastosphere, at the junction between the cells which will 

 be invaginated and the epiblast cells. In the bilateral blastosphere, from which he 

 holds that all the higher forms (Bilateralia) have originated, these cells had a 

 bilateral arrangement, and thus the bilateral origin of the mesoblast is explained. 

 The origin of the mesoblast from the lips of the blastopore is explained by the 

 position of its mother-cells in the blastosphere. It need scarcely be said that the 

 views already put forward as to the probable mode of origin of the mesoblast, 

 founded on the analogy of the Ccelenterata, are quite incompatible with Rabl's 

 theories. 



