The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . ta); it is a groove-likedepression of the outer skin (epidermis). At the back of thehead, near the after-brain, at the upper end of the secondgill-opening, a little wart-like thickening of the horn-platearises on each side (Figs. 24^6, A, fl; 248, g). This deepensinto a small groove, and separates from the outer-skin, justas does the lens of the eye. (Cf. p. 253.) A small vesiclefilled with fluid, the primitive ear-vesicle, is thus formedon each side, immediately
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . ta); it is a groove-likedepression of the outer skin (epidermis). At the back of thehead, near the after-brain, at the upper end of the secondgill-opening, a little wart-like thickening of the horn-platearises on each side (Figs. 24^6, A, fl; 248, g). This deepensinto a small groove, and separates from the outer-skin, justas does the lens of the eye. (Cf. p. 253.) A small vesiclefilled with fluid, the primitive ear-vesicle, is thus formedon each side, immediately below the hom-plate of the backpai-t of the head; this is also called the primary laby-rmth (Plates VI. and VII.). As this separates from itsoriginal site, the horn-plate, and grows inward and down-ward in the skull, it changes from a globular to a pear-shaped form (Figs. 246, B, Lv ] 249, o). The outer part has 264 THE EVOLUTION OF MAN. elongated into a thin stalk, which at first opens outward ina narrow canal. (Cf. Fig. 137,/, vol. i. p. 382.) This is calledthe appendage of the labyrinth recessus labyrinthi. Fig. 24-6, Ir).. ^.
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