The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . entery-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
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . entery-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 IV. Fig. b,k). Thecells of this germ-plate, or sexual plate (lamella sexualis) areessentially distinguished by their cylindrical form and bytheir chemical constitution from the other cells of thecoelom; they are of quite different significance from the fiat cells of the serous coelom-epithelium which line the5a 400 THE EVOLUTION OF MAX. romainder of the body-cavity (cceloina). Of tliese latter—the true coelom-cells—those which invest the intestinaltube and the mesentery ( endocoilar ) originate from the. c~ Fig. 316.—Transverse section through the pelvic resfion and the hindlimbs of an embryo Chick in the fourth day of incubation, enlarged about40 times : h, horn-plate ; u, medullary tube; n, canal of the medullarytube; u, primitive kidneys; x, notochord; e, hind limbs ; h, allantois canalin ventral wall; t, aorta; v, cardinal veins ; a, intestine; d, intestinal-glandular layer; /, intestinal-fibrous layer; ^, germ-epithelium ; r, dorsalmuscles; c, body-cavity, or Ccelom. (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 coelom-cells, and DIFFERENTIATION
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