348 ORIGIN OF THE MESOBLAST. 



ation of the mesoblast as a distinct layer by the process 

 already explained, the two primary layers lost for the 

 most part the capacity they primitively possessed of 

 giving rise to muscular and connective-tissue diffe- 

 rentiations 1 , to the epithelium of the excretory organs, 

 and to generative cells. (2) That the mesoblast through- 

 out the triploblastic Metazoa, in so far as these forms 

 have sprung from a common triploblastic ancestor, is 

 an homologous structure. 



The second proposition follows from the first. The mesoblast 

 can only have ceased to be homologous throughout the triplo- 

 blastica by additions from the two primary layers, and the 

 existence of such additions is negatived by the first proposition. 



These two propositions, which hang together, are possibly 

 only approximately true, since it is quite possible that future 

 investigations may shew that differentiations of the two primary 

 layers are not so rare as has been hitherto imagined. 



Ranvier 2 finds that the muscles of the sweat-glands are developed from 

 the inner part of the layer of epiblast cells, invaginated to form these 

 glands. 



Gotte 3 describes the epiblast cells of the larva of Comatula as being at a 

 certain stage contractile and compares them with the epithelio-muscular 

 cells of Hydra. These cells would appear subsequently to be converted 

 into a simple cuticular structure. 



It is moreover quite possible that fresh differentiations from 

 the two primary layers may have arisen after the triploblastic 

 condition had been established, and by the process of simplifi- 

 cation of development and precocious segregation, as Lankester 

 calls it, have become indistinguishable from the normal meso- 

 blast. In spite of these exceptions it is probable that the major 

 part of the muscular system of all existing triploblastic forms 

 has been differentiated from the muscular system of the ancestor 

 or ancestors (if there is more than one phylum) of the triplo- 



1 The connective-tissue test of the Tunicata, though derived from the epiblast, is 

 not really an example of such a differentiation. 



* M. L. Ranvier. " Sur la structure des glandes sudoripares." Comptes Rendus, 

 Dec. 29, 1879. 



3 A. Gotte, "Vergleich. Entwick. d. Comatula mediterranea." Archiv f. mikr. 

 Anat. vol. xii. p. 597. 



