The whole dorsal wall of the posterior part of the archenteron is converted into the notochord (fig. 181 _ch_), and the lateral walls into the mesoblast (_me_); so that the original lumen of the posterior part of the archenteron ceases to be bounded by hypoblast cells, and disappears as such. Part of the ventral wall remains as a solid cord of cells (_al´_) The anterior part of the archenteron in front of the notochord passes wholly into the permanent alimentary tract. The derivation of the mesoblast from the lateral walls of the posterior part of the archenteron is clearly comparable with the analogous process in Amphioxus. Vertebrata. In turning from Amphioxus to the true Vertebrata we find no form in which diverticula of the primitive alimentary tract give rise to the mesoblast. There is reason to think that the type presented by the Elasmobranchii in the formation of the mesoblast is as primitive as that of any other group. In this group the mesoblast is formed, nearly coincidently with the hypoblast of the dorsal wall of the mesenteron, as two lateral sheets, one on each side of the middle line (fig. 182 _m_). These two sheets are at first solid masses; and their differentiation commences in front and is continued backwards. After their formation the notochord arises from the axial portion of the hypoblast (which had no share in giving rise to the two mesoblast plates) as a solid thickening (fig. 183 _ch´_), which is separated from it as a circular rod. Its differentiation, like that of the mesoblastic plates, commences in front. The mesoblast plates subsequently become divided for their whole length into two layers, between which a cavity is developed (fig. 184). The dorsal parts of the plates become divided by transverse partitions into somites, and these somites with their contained cavities are next separated from the more ventral parts of the plates (fig. 185 _mp_). In the somites the cavities become eventually obliterated, and from their inner sides plates of tissue for the vertebral bodies (fig. 186 _Vr_) are separated; while the outer parts, consisting of two sheets, containing the remains of the original cavity, form the muscle-plates (_mp_). [FIG. 182. TWO TRANSVERSE SECTIONS OF AN EMBRYO PRISTIURUS OF THE SAME AGE AS FIG. 17. A. Anterior section. B. Posterior section. _mg._ medullary groove; _ep._ epiblast; _hy._ hypoblast; _n.al._ cells formed round the nuclei of the yolk which have entered the hypoblast; _m._ mesoblast. The sections shew the origin of the mesoblast.] [FIG. 183. THREE SECTIONS OF A PRISTIURUS EMBRYO SLIGHTLY OLDER THAN FIG. 28 B. The sections shew the development of the notochord. _Ch._ notochord; _Ch´._ developing notochord; _mg._ medullary groove; _lp._ lateral plate of mesoblast; _ep._ epiblast; _hy._ hypoblast.] [FIG. 184. TRANSVERSE SECTION THROUGH THE TAIL-REGION OF A PRISTIURUS EMBRYO OF THE SAME AGE AS FIG. 28 E.