340 DEVELOPMENT OF ELASMOBRANCH FISHES. 



apparent rather than real exceptions have also been pointed 

 out. The Mollusks, however, remain as a large group, in which 

 we as yet know very little as to the formation of the mesoblast. 



Dr Rabl 1 , 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 2 and Dr 

 Fol 3 are equally inconclusive for this purpose, for, though they 

 contain figures of elongated 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 they arose. The most 

 accurate observations which we have on this question are those 

 of Professor Bobretzky 4 . In Nassa he finds that the three 

 embryonic layers are all established during segmentation. The 

 outermost and smallest cells form the epiblast, somewhat larger 

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

 hypoblast. 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 con- 

 sideration. 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 5 . The diverticula 



1 Jenaische Zeitschrift, Vol. IX. 



2 Quart. Jl. of Micros. Science, Vol. xxv. 1874, and Phil. Trans. 1875. 



3 Archives dt Zoologie, Vol. iv. 



4 Archiv f. Micr. Anat. Vol. XIII. 



5 The recent researches of Selenka, Zeitschrift f. \Viss. Zoologie, Vol. XXVII. 1876, 

 demonstrate that in Echinoderms the muscles are derived froni the cells first split off 



