The evolution of man : a popular exposition of the principal points of human ontogeny and phylogeny . a hoodrenders this process of inversion, which is sometimes hardto explain, more clearly understood. The rudiment of thevitreous body (corpus vitreum) is at first very incon- 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 humanembryo of four weeks; 100 timesenlarged (after Koelliker)
The evolution of man : a popular exposition of the principal points of human ontogeny and phylogeny . a hoodrenders this process of inversion, which is sometimes hardto explain, more clearly understood. The rudiment of thevitreous body (corpus vitreum) is at first very incon- 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 humanembryo 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 Vitreous body be-hind the lens); r, retina (inner,thicker, inverted lamella of theprimary eye-vesicle) ; a. pigment 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 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 thevitreous 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 truevascular 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 (the stalk of DEVELOPMENT OF THE EYE. 257 the primar
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Keywords: ., book, bookcentury1800, bookdecade1870, booksubjectembryologyhuman
