The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . ens and below it, a ledge-likeprocess of the leather-plate arises (Fig. 243, g), which invertsthe primary eye-vesicle (now shaped like a cup) from below,and presses in between the lens (l) and the retina (r)Thus the primary eye-vesicle assumes the form of a opening of this hood, answering to the face, is coveredby the lens; but the opening, through which the neckwould pass, answers to the indentation through which theleather-skin passes in between the lens and the retina (theinner wall
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogene . ens and below it, a ledge-likeprocess of the leather-plate arises (Fig. 243, g), which invertsthe primary eye-vesicle (now shaped like a cup) from below,and presses in between the lens (l) and the retina (r)Thus the primary eye-vesicle assumes the form of a opening of this hood, answering to the face, is coveredby the lens; but the opening, through which the neckwould pass, answers to the indentation through which theleather-skin passes in between the lens and the retina (theinner wall of the hood). The space within this secondaryeye-vesicle is almost filled by the vitreous body, whichanswers to the head wrapped in this hood. The liood itselfis, properly speaking, double: the inner hood itself is theretina, and the outer one, directly surrounding the former,is the pigment membrane. The comparison with a hoodrenders this process of inversion, which is sometimes hardto explain, more clearly understood. The rudiment of thevitreous body {corpius vitreum) is at first very incon-50. 256 THE EVOLUTION OF MAN. siderable (Fig. 243, g), and the retina disproportionallythick. As the former expands, the latter becomes muchthinner, till at last the retina appears only as a very delicate Fig. 243.—Horizontal tranaversesection through the eye of a humanembryo of four weeks; 100 fcimeaenlarged (after Koelliker): t, lens(the dark wall of which is equal tothe diameter of the central cavity) ;g, vitreous body (connected with theleather-plate by a stalk, g) j v, vas-cular loop (penetrating through thestalk {g) into the vitreous body be-hind the lens); r, retina (inner,thicker, inverted lamella of theprimary eye-vesicle); a, pigiuent membrane (outer, thinner, uninvertedlamella of the same); h, intermediate space between the retina and thepigment membrane (remnant of the cavity of the primary eye-vesicle). coat of the thick, almost globular vitreous body, which fillsthe greater
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Keywords: ., bookcentury1800, booksubjectembryologyhu, booksubjecthumanbeings
