Dr Rabl[216], who seems recently to have studied the development of Lymnæus 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[217] and Dr Fol[218] 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[219]. 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. Footnote 216: _Jenaische Zeitschrift_, Vol. IX. Footnote 217: _Quart. Jl. of Micros. Science_, Vol. XXV. 1874, and _Phil. Trans._ 1875. Footnote 218: _Archives de Zoologie_, Vol. IV. Footnote 219: _Archiv f. Micr. Anat._ Vol. XIII. 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 consideration. 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[220]. The diverticula from the alimentary cavity form the water-vascular system and the somatic and splanchnic layers of mesoblast. _The cavity of the diverticula after the separation of the water-vascular system, forms the body-cavity. The outer lining layer of the cavity forms the somatic layer of mesoblast and the voluntary muscles; the inner lining layer the splanchnic mesoblast which unites with the epithelium of the alimentary tract._ Though this fundamental arrangement would seem to be universal amongst Echinoderms, considerable variations of it are exhibited in different groups.