The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . drenders this process of inversion, which is sometimes hardto explain, more clearly understood. The rudiment of thevitreous body (corpus 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 transversesection through the eye of a hnmanembryo of four weeks; 100 tim
The evolution of man: a popular exposition of the principal points of human ontogeny and phylogenyFrom the German of Ernst Haeckel . drenders this process of inversion, which is sometimes hardto explain, more clearly understood. The rudiment of thevitreous body (corpus 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 transversesection through the eye of a hnmanembryo of four weeks; 100 timesenlarged (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) ; v, vas-cular loop (penetrating through thestalk {g) into the vitr-eous body be-hind the lens); r, retina (inner,thicker, inverted lamella of theprimaiy eye-vesicle); a, pigment membrane (outer, thinner, uninvertedlamella of the same); hy intermediate space between the retina and thepigment membi-aue (remnant of the cavity of the eye-vesicle).. coat of the tlnck, almost globular vitreous body, which fillsthe greater part of the secondary eye-vesicle. The outerlayer of the vitreous body changes into a highly vascularcapsule, the vessels of which afterwards disappear. The slit-like passage through which the rudiment of thevitieous body grows from below in between the lens andthe retina, of course causes a break in the retina and thepigment-membrane. This break, which appears on the innersurface of the vascular membrane as a colourless streak, hasbeen inaptly called the choroidal cleft, though the truev^ascular membrane is not cleft at all at this point (, sp, 235, sp, p. 243). A thin process of the vitreous bodypasses inward on the under surface of the optic nerve, whichit inverts in the same way as the primary eye-vesicle wasinverted. The hollow cylindrical optic nerve (tlie stalk of DEVE
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Keywords: ., bookcentury1800, booksubjectembryology, booksubjectembryologyhum
