5. The mesoblast is split off from the hypoblast. Nemertea.--Some of the types without a metamorphosis. Mollusca.--Scaphopoda. It is derived from the lateral and ventral cells of the hypoblast. Gephyrea.--Phascolosoma. Vertebrata.--In most of the Ichthyopsida the mesoblast is derived from the hypoblast (fig. 213). In some types (_i.e._ most of the Amniota) the mesoblast might be described as originating at the lips of the blastopore (primitive streak). 6. The mesoblast is derived from both germinal layers. Tracheata.--Araneina (fig. 214). It is derived partly from cells split off from the epiblast and partly from the yolk-cells; but it is probable that the statement that the mesoblast is derived from both the germinal layers is only formally accurate; and that the derivation of part of the mesoblast from the yolk-cells is not to be interpreted as a derivation from the hypoblast. Amniota.--The derivation of the mesoblast of the Amniota from both the primary germinal layers is without doubt a secondary process. The conclusions to be drawn from the above summary are by no means such as might have been anticipated. The analogy of the Coelenterata would lead us to expect that the mesoblast would be derived partly from the epiblast and partly from the hypoblast. Such, however, is not for the most part the case, though more complete investigations may shew that there are a greater number of instances in which the mesoblast has a mixed origin than might be supposed from the above summary. [FIG. 211. SECTIONS OF AN AMPHIOXUS EMBRYO AT THREE STAGES. (After Kowalevsky.) A. Section at gastrula stage. B. Section of a somewhat older embryo. C. Section through the anterior part of still older embryo. _np._ neural plate; _nc._ neural canal; _mes._ archenteron in A, and mesenteron in B and C; _ch._ notochord; _so._ mesoblastic somite.] I have attempted to reduce the types of development of the mesoblast to six; but owing to the nature of the case it is not always easy to distinguish the first of these from the last four. Of the six types the second will on most hands be admitted to be the most remarkable. The formation of hollow outgrowths of the archenteron, the cavities of which give rise to the body cavity, can only be explained on the supposition that the body cavity of the types in which such outgrowths occur is derived from diverticula cut off from the alimentary tract. The lining epithelium of the diverticula--the peritoneal epithelium--is clearly part of the primitive hypoblast, and this part of the mesoblast is clearly hypoblastic in origin. [FIG. 212. SECTIONS THROUGH THE OVUM OF LEPTOPLANA TREMELLARIS IN THREE STAGES OF DEVELOPMENT. (After Hallez.) _ep._ epiblast; _m._ mesoblast; _hy._ yolk-cells (hypoblast); _bl._ blastopore.]