The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . mordial skulL In the posterior half, on the contrary,the primitive vertebral plate breaks up into a number ofhomologous cube-shaped pieces, lying one behind the other,these are the several primitive vertebrae. The numberof these is at first very small, but soon increases, as thegerm grows in the posterior direction (Figs. 258-2G0, uw).The first and earliest primitive vertebrae are the foremostneck-vertebrae ; the posterior neck-vertebrae then originate ;then the
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . mordial skulL In the posterior half, on the contrary,the primitive vertebral plate breaks up into a number ofhomologous cube-shaped pieces, lying one behind the other,these are the several primitive vertebrae. The numberof these is at first very small, but soon increases, as thegerm grows in the posterior direction (Figs. 258-2G0, uw).The first and earliest primitive vertebrae are the foremostneck-vertebrae ; the posterior neck-vertebrae then originate ;then the anterior breast-vertebrae, etc. The lowest of thetail-vertebrae arise last. This successive ontoi::enetic growthof the vertebral column in a direction from front to rearmay be explained phylogenetically by regarding the many-membered vertebrate body as a secondary product, whichhas originated from an originally inarticulate parent-formby progressive metameric development, or as the many-membered Worms (Earth-worm, Leech) and the closely allied Arthropods (Crabs, Insects) originally52 288 THE EVOLUTION OF Figs. 258-2()0.—Lyre-shaped germ-shield of a Chick, in three consecutivestages of development; seen from the dorsal side ; enlarged about twentytimes. Fig. 258, with six pairs of primitive vertebrae. The brain is a sim-ple bladder (hh). The spinal furrow from x remains wide open ; behind, atz, it is much enlarged, mi?, Marrow-plates; sp, side-plates ; y, limit be-tween the pharynx cavity (sh) and the head-intestine (vd). Fig. 259, withten pairs of primitive vertebrae. The brain has separated into threebladders: v, fore-brain; m, mid-brain; /), hind-brain; c, heart; dv, yelk-veins. The spinal furrow is still wide open (z). nip, 260, with sixteen pairs of primitive vertebrae. The brain has separated PRIMITIVE VERTEBRA. 289 into five bladders: v, fore-brain; 2, twixt-brain; m, mid-brain; h^ hind-hrain; n, after-brain; a, eye-vesicles; g
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Keywords: ., bookcentury1800, booksubjectembryology, booksubjectembryologyhum
