The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . as also undoubtedly thecase in all extinct Gastrseads. In all Primitive Animals(Protozoa) the nerve-system is, of course, unrepresented, forthese have not as yet attained to the development of germ-layers. In considering the individual development of the nerve-system in the human embryo, we must fiist of all start fromthe important fact already mentioned, that the first rudi-ment of the system is the simple medullary tube, whichdetaches itself from the outer germ-layer along the middleline of t
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . as also undoubtedly thecase in all extinct Gastrseads. In all Primitive Animals(Protozoa) the nerve-system is, of course, unrepresented, forthese have not as yet attained to the development of germ-layers. In considering the individual development of the nerve-system in the human embryo, we must fiist of all start fromthe important fact already mentioned, that the first rudi-ment of the system is the simple medullary tube, whichdetaches itself from the outer germ-layer along the middleline of the lyre-shaped primitive germ. We found (, vol. i. p. 298) that the rectilineal primitive groove, ordorsal furiow, first arises in the centre of the lyre-shapedgerm-disc. On each side of this rise the two parallel dorsalor medullary swellings. The free margins of these bend to-wards each other, coalesce, and form the closed medullaiy tube(Figs. 88-93, vol. i. pp. 300-309). At first this tube lies directlyunder the horn-plate; it is, however, afterwards situate 2l8 THE EVOLUTION OF MAN,. Figs. 221-223.—Lyre-shaped (or sole-shaped) germ-shield of a Chick, inthree consecutive stages of evolution, seen from the dorsal surface: abouttwenty times enlarged. Fig. 221, with six pairs of primitive brain a simple bladder (hh). The medullary furrow is wide open fromthe pointic, very wide at z. -mp. Marrow (or medullary) plates; sp, side-plates ; y, boundary between the throat cavity (sh) and the head-intestine(vd). Fig. 222, with ten pairs of primitive vertebrae. The brain consists ofthree bladders : v, fore-brain; m, mid-brain ; h, hind-brain, c, Heart; dv,yelk-veins. The medullary furrow is wide open behind (z). mp, Marrow-plates. Fig. 223, with sixteen pairs of primitive vertebrae. The brainconsists of five bladders: v, fore-brain; z, twixt-brain; m, mid-brain; h,hind-brain ; n, after-brain, a, Eye-vesicles ; g^ ear-vesicles ; c, heart; dv,yelk-veins ; mp, marr
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Keywords: ., bookcentury1800, booksubjectembryologyhu, booksubjecthumanbeings
