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.
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. 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.
df. dorsal fin; sp.c. spinal cord; pp. body cavity; sp. splanchnic layer of mesoblast; so. somatic layer of mesoblast; mp´. commencing differentiation of muscles; ch. notochord; x. subnotochordal rod arising as an outgrowth of the dorsal wall of the alimentary tract; al. alimentary tract.
The undivided ventral portion gives rise to the general somatic and splanchnic mesoblast ([fig. 185]), and the cavity between its two layers constitutes the body cavity. The originally separate halves of the body cavity eventually meet and unite in the ventral median line throughout the greater part of the body, though in the tail they remain distinct and are finally obliterated. Dorsally they are separated by the mesentery. From the mesoblast at the junction of the dorsal and ventral parts of the primitive plates is formed the urinogenital system.