122 DERIVATION OF THE MESOBLAST. 



Dr Rabl ^, who seems recently to have studied the develop- 

 ment of Lymnaeus by means of sections, gives some figures 

 shewing the origin of the mesoblast ; they are, however, too 

 diagrammatic to be of much service in settling the present 

 question, and the memoirs of Professor Lankester'^ and Dr 

 FoP are equally inconclusive for this purpose, for, though they 

 contain fio-ures of elonorated and branched mesoblast cells 

 passing from the epiblast to the hypoblast, no satisfactory 

 representations are given of the origin of these cells. I have 

 myself observed in embryos of Turbo or Trochus similar 

 elongated cells to those figured by Lankester and Fol, but was 

 unable clearly to determine whence the}^ arose. The most accu- 

 rate observations which we have on this question are those of 

 Professor Bobretzky ^ In Nassa he finds that the three embry- 

 onic layers are all established during segmentation. The outer- 

 most and smallest cells form the epiblast, somewhat larger cells 

 adjoining these the mesoblast, and the large yolk-cells the hypo- 

 blast. These observations do not, however, demonstrate from 

 which of the primary layers the mesoblast is derived. 



The evidence at present existing is clearly in favour of the 

 mesoblast being, in almost all groups of animals, developed from 

 the hypoblast, but strong as this evidence is, it has not its full 

 weight unless the actual manner in which the mesoblast is in 

 many groups derived from the hypoblast, is taken into consider- 

 ation. The most important of these are the Echinoderms, 

 Brachiopods and Sagitta. 



In the Echinoderms the mesoblast is in part formed by cells 

 budded off from the hypoblast, the remainder, however, arises as 

 one or more diverticula of the alimentary tract. From the separate 

 cells first budded off there are formed the cutis, part of the 

 connective tissue and the calcareous skeleton^. The diverticula 

 from the alimentary cavity form the water-vascular system and 

 the somatic and splanchnic layers of mesoblast. The cavity of 



1 Jenaische Zeitschrift, Vol. ix. 



2 Quart. Jl. of Micros. Science, Vol. xxv. 187-1, and Phil. Trans. 1875. 



3 Archives de Zoologie, Vol. iv. 



4 Archiv f. Micr. Anat. Vol. xiii. 



^ The recent researches of Selenka, Zeitschrift f. Wiss. ZooJo(jie, Vol. xxvii. 

 1876, demonstrate that in Echinoderms the muscles are derived from the cells 

 fast split oif from the hypoblast, and that the diverticula only form the water- 

 vascular system and the epithelial lining of the body-cavity. 



