The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . esentery-plate) (Fig. 318, mp, p. 408).At this very important point in the coelom-wall, where theendocoelar (or visceral coelom-epithelium) merges into theexocoelar (or parietal coelom-epithelium), in the embryo ofMan and the other Skulled Animals a small aggregation ofcells becomes visible, at a very early period, and this, accord-ing to Waldeyer,^^^ we may call the germ-epithelium, or(corresponding with the other plate-shaped rudiments oforgans) the sexual plate (Fig. 316, ^; Plate TV. Fig. 5
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . esentery-plate) (Fig. 318, mp, p. 408).At this very important point in the coelom-wall, where theendocoelar (or visceral coelom-epithelium) merges into theexocoelar (or parietal coelom-epithelium), in the embryo ofMan and the other Skulled Animals a small aggregation ofcells becomes visible, at a very early period, and this, accord-ing to Waldeyer,^^^ we may call the germ-epithelium, or(corresponding with the other plate-shaped rudiments oforgans) the sexual plate (Fig. 316, ^; Plate TV. Fig. 5,h). Thecells of this germ-plate, or sexual plate (lamella sexualis) ai-eessentiaUy distinguished by their cylindrical form and bytheir chemical constitution from the other cells of thecoelom; they are of quite difierent significance n^om the flatcells of the serous ccelom-epithelium which line the 59 400 THE EVOLUTION OF MAN. remainder of the body-cavity (coeloma). Of these latter—the true eoelom-cells—those which invest the intestinaltube and the mesentery ( endocoelar ) originate from the. Fig. 316.—Transverse section through the pelvic region and the hindlimbs of an embryo Chick in the fonrth day of incubation, enlarged about40 times : h, horn-plate ; tv, medullary tube; n, canal of the medullarytube; u, primitive kidneys ; x, notochord; e, hind limbs ; b, allantois canalin ventral wall; t, aorta; v, cardinal veins ; a, intestine; d, intestinal-glandular layer; /, intestinal-fibrous layer; g, germ-epithelium ? r, dorsalmuscles; c, body-cavity, or Coelom. {After Waldeyer.) intestinal-fibrous layer (in Fig. 5, Plate IV., coloured red);those which line the inner surface of the external wall ofthe abdomen ( exocodar ) are, on the contrary, the productof the skin-fibrous layer (coloured blue in Fig. 5, Plate IV.);but the sexual cells which make their appearance at theboundary line between the two forms of ccelom-cells, and DIFFERENTIATION OF THE SEXES. 4OI which insert the
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Keywords: ., bookcentury1800, booksubjectembryologyhu, booksubjecthumanbeings
