The evolution of man : a popular exposition of the principal points of human ontogeny and phylogeny . ynamoebium), of the phylogenetic existence of whichwe yet have evidence in the ontogenetic developmentalform of the mulberry-germ (Morula), the several membersof the community began to divide the work of life, theywere first obliged to engage in two separate tasks. Onehalf modified into nutritive cells, enclosing a digestivecavity, the intestinal canal; the other half, on the contrary,developed into covering cells, forming the outer cover-ing of this intestinal canal, and, at the same time, of
The evolution of man : a popular exposition of the principal points of human ontogeny and phylogeny . ynamoebium), of the phylogenetic existence of whichwe yet have evidence in the ontogenetic developmentalform of the mulberry-germ (Morula), the several membersof the community began to divide the work of life, theywere first obliged to engage in two separate tasks. Onehalf modified into nutritive cells, enclosing a digestivecavity, the intestinal canal; the other half, on the contrary,developed into covering cells, forming the outer cover-ing of this intestinal canal, and, at the same time, of thewhole body. Thus arose the first two germ-layers : the PRIMITIVE INTESTINAL CANAL. 313 inner, nutritive, or vegetative layer, and the outer, covering,or animal layer. If we try to construct for ourselves an animal body ofthe simplest conceivable form, possessing such a primitiveintestinal canal, and the two primary germ-layers formingits wall, the result is necessarily the very remarkablegerm-form of the gastrula, which we have shown to existin wonderful uniformity throughout the whole animal. Fig. 274. —Gastrula of a Chalk-sponge (Olynthus) : A, from outside;Bj in longitudinal section through the axis ; g, primitive intestine ; 0, primi-tive mouth ; i, intestinal layer, or entoderm ; e, skin-layer, or exoderm. series: in the Sponges, Sea-nettles (Acalephce), Worms,Soft-bodied Animals (Mollusca), Articulated Animals {Arthvo-poda), and Vertebrates (Figs. 174-179, p. 65). In all thesevarious animal tribes the gastrula reappears in the sameentirely simple form (Fig. 274). Its whole body is reallymerely the intestinal canal ; the simple cavity of the body,the digestive intestinal cavity, is the primitive intestine 314 THE EVOLUTION OF MAN. (protogaster, g); its simple opening, the primitive mouth(jorotostoma, 6), is at once mouth and anus; and the twocell-strata which compose its wall, are the two primarygerm-layers: the inner, the nutritive, or vegetative germ-layer, is the
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