The organs invariably derived, in the triploblastic forms, from the mesoblast, are the vascular and lymphatic systems, the muscular system, and the greater part of the connective tissue and the excretory and generative (?) systems. On the other hand, the nervous systems (with a few possible exceptions) and organs of sense, the epithelium of most glands, and a few exceptional connective-tissue organs, as for example the notochord, are developed from the two primary layers. The fact of the first-named set of organs being invariably derived from the mesoblast points to the establishment of the two following propositions:--(1) That with the differentiation 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 differentiations[127], to the epithelium of the excretory organs, and to generative cells. (2) That the mesoblast throughout the triploblastic Metazoa, in so far as these forms have sprung from a common triploblastic ancestor, is an homologous structure. [127] The connective-tissue test of the Tunicata, though derived from the epiblast, is not really an example of such a differentiation. The second proposition follows from the first. The mesoblast can only have ceased to be homologous throughout the triploblastica 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[128] 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. [128] M. L. Ranvier. "Sur la structure des glandes sudoripares." _Comptes Rendus_, Dec. 29, 1879. Götte[129] 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. [129] A. Götte, "Vergleich. Entwick. d. Comatula mediterranea." _Archiv f. mikr. Anat._ vol. XII. p. 597.