288 



ORIGIN OF THE MESOBLAST. 



These two -propositions, which hang together, are possibly only 

 approximately true, since it is quite possible that future investiga- 

 tions may shew that differentiations of the two primary layers are 

 not so rare as has been hitherto imagined. 



Ranvier 1 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 Comatulaas being at a 

 certain stage contractile and compares them with the epithelio-muscular 

 ft-lls 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 primaiy layers may have arisen after the triploblastic condition 

 had been established, and by the process of simplification of develop- 

 ment and precocious segregation, as Lankester calls it, have become 

 indistinguishable from the normal mesoblast. 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 triploblastica. In the case of other tissues there 

 are a few instances which might be regarded as examples of an 

 organ primitively developed in one of the two primary layers having 

 become secondarily carried into the mesoblast. The notochord has 

 sometimes been cited as such an organ, but, as indicated in a 

 previous chapter, it is probable that its hypoblastic origin can always 

 be demonstrated. 



CjJ 



me 



Vic,. 20(5. EPIBOLIC GASTKULA OF BONELLIA. (After Spengel.) 



A. Stage when the four hypoblast cells are nearly enclosed. 



B. Stage after the formation of the mesoblast has commenced by an infolding of 

 the lips of the blastopore. 



cp. epiblast; inc. mesoblast; 11. hlastopore. 



1 M. L. Ranvier, "Sur la structure des glandes sudoiipares." Coiinttes AVm/i/x, 

 Dec. '29, 179. 



- A. (rfitte, "Vergldch. Entwick. d. Comatula mediterranean" An-lih- I'. mil;r. 

 Anat. vol. XIT. p. .V.I7. 



