The development of the human body; a manual of human embryology . Fig. 252.—Early Stage in the Development of the Lens in a Rabbit nucleated layer to the left is the ectoderm and the thicker lens epithelium, below which is the outer wall of the optic evagination; above and below between the two is mesenchyme.—(Rabl.) In the mean time further changes have been taking placein the lens. The ectodermal depression which represents 478 THE DEVELOPMENT OF THE HUMAN BODY. it gradually deepens to form a cup, the lips of which ap-proximate and finally meet, so that the cup is convertedinto a
The development of the human body; a manual of human embryology . Fig. 252.—Early Stage in the Development of the Lens in a Rabbit nucleated layer to the left is the ectoderm and the thicker lens epithelium, below which is the outer wall of the optic evagination; above and below between the two is mesenchyme.—(Rabl.) In the mean time further changes have been taking placein the lens. The ectodermal depression which represents 478 THE DEVELOPMENT OF THE HUMAN BODY. it gradually deepens to form a cup, the lips of which ap-proximate and finally meet, so that the cup is convertedinto a vesicle which finally separates completely from theectoderm (Fig. 254), much in the same way as the otocystdoes. As the lens vesicle is constricted off, the surround-ing mesodermal tissue grows in to form a layer betweenit and the overlying ectoderm, and a split appearing in thelayer divides it into an outer thicker portion, which rep-resents the cornea, and an inner thinner portion, which. Fig. 253.—Reconstruction of the Brain of an Embryo of FourWeeks, showing the Chorioid Fissure.—(His.) covers the outer surface of the lens and becomes highlyvascular. The cavity between these two portions repre-sents the anterior chamber of the eye. The cavity of theoptic cup has also become filled by a peculiar tissue whichrepresents the vitreous humor, while the mesodermal tissuesurrounding the cup condenses to form a strong invest-ment for it, which is externally continuous with thecornea, and at about the sixth week shows a differentia-tion into an inner vascular layer, the chorioid coat, and anouter denser one, which becomes the sclerotic coat. THE LENS. 479 The various processes resulting in the formation of theeye, which have thus been rapidly sketched, may now beconsidered in greater detail. The Development of the Lens.—When the lens vesicle iscomplete, it forms a more or less spherical sac lying be-
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
