The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . g) lie amoeboid eggs (e). The skin-layer (h) belowis encrusted with grains of sand, above with sponge-spicnles. opening at the top is the mouth-opening (Fig. 181, m).The two cell-strata forming the wall of the pouch are the 6S THE EVOLUTION OF MAN. two primary germ-layers. These most simple Plant-Animalsdiffer from the gastrula principally in the fact that theformer are attached by one end (that opposite to the mouth-opening) to the bottom of the sea, while the latter arefree. Moreover, the cel
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . g) lie amoeboid eggs (e). The skin-layer (h) belowis encrusted with grains of sand, above with sponge-spicnles. opening at the top is the mouth-opening (Fig. 181, m).The two cell-strata forming the wall of the pouch are the 6S THE EVOLUTION OF MAN. two primary germ-layers. These most simple Plant-Animalsdiffer from the gastrula principally in the fact that theformer are attached by one end (that opposite to the mouth-opening) to the bottom of the sea, while the latter arefree. Moreover, the cells of the skin-layer are coalescent andhave included many foreign bodies, such as sponge-spicules,sand-grains, etc., which serve to support the body-wall(Fig. 180). The intestinal layer, on the other hand, con-sists merely of a stratum of ciliated cells (Fig 181, d).When the Haliphysema is sexually mature, individual cellsof its entoderm assume the character of female egg-cells;on the other hand, individual cells of its exoderm becomemale seed-cells; the fertilization of the former by the latter. Figs. 182, of a Sponge (Olynthus). Fig. 182, from the ont-side: Fig. 188, in lono-itudinal secHon : g, primitive intestine ; o, primitivemouth; i, intestinal layer ; e, skin-layer. REPRODUCTION IN THE GASTRJ5ADS. 69 takes place directly through the stomach-cavity. A truepalingenetic gastrula (Fig. 174) develops, just as in theMonoxenia (Fig. 171), from the fertilized egg. This swimsabout for a time in the sea, then attaches itself, and ia thisstate resembles a simple young-form, which occurs in thecourse of the evolution of many other Plant-Animals, andwhich is called the ascula (Figs. 182, 183). In consequenceof the absorption of foreign bodies by the exoderm, itbecomes the Haliphysema. When we consider that there is no other importantdifference between the free-swimming gastrula and thisattached, simplest Plant-animal, we are fairly justified instating that in the simple
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