294 



ORIGIN OF THE MESOBLAST. 



of the mesoblast being derived from amoeboid cells which spring 

 from the walls of the archenteron before the origin of the vaso- 

 peritoneal outgrowths (figs. 199 and 210). 



Reserving for the moment the question as to what conclusions 

 can be deduced from the above facts as to the origin of the meso- 

 blast, it is important to determine how far the facts of embryology 

 warrant us in supposing that in the whole of the triploblastic forms 

 the body cavity originated from the alimentary diverticula. There 

 can be but little doubt that the mode of origin of the mesoblast in 

 many Vertebrata, as two solid plates split off from the hypoblast, in 



which a cavity is secondarily 

 developed, is an abbreviation of 

 the process observable in Am- 

 phioxus ; but this process ap- 

 proaches in some forms of Ver- 

 tebrata to the ingrowth of the 

 mesoblast from the lips of the 

 blastopore. 



It is, therefore, highly pro- 

 bable that the paired ingrowths 

 of the mesoblast from the lips 

 of the blastopore may have been 

 in the first instance derived from 

 a pair of archenteric diverticula. 

 This process of formation of the 

 mesoblast is, as may be seen by 

 reference to the summary, the 

 most frequent, including as it 

 does the Chgetopoda, the Mol- 

 lusca, the Arthropoda, &c.^ 



While there is no difficiilty in 

 the view that the body cavity may 

 have originated from a pair of enteric 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 ali- 

 mentary diverticula. 



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

 doubt, passible that a body cavity may have existed in the Platyelminthes, 

 and become lost ; and the case of the Discophora, which in their muscular 



^ The wide occurrence of this process was first pointed out by Eabl. He holds, 

 however, a peculiar modification of the gastra?a 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 wiU 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. 



Fig. 213. Two sections op a young 

 Elasmobbanch embryo, to shew the me- 

 soblast SPLIT OIF AS TWO LATERAL MASSES 

 FROM THE HYPOBLAST. 



mg. medullary groove; ep. epiblast; vi. 

 mesoblast; hy. hypoblast; n.al. cells 

 formed around the nuclei of the yolk which 

 have entered the hypoblast. 



