The development of the human body; a manual of human embryology . n with theectoderm of the side of the head, and this, over the areaof contact, becomes thickened and then depressed to formthe beginning of the future lens (Fig. 252). As the result of the depression of the lens ectoderm, theouter wall of the optic bulb becomes pushed inward to-ward the inner wall, and this invagination continuing untilthe two walls come into contact, the bulb is transformedinto a double-walled cup, the optic cup, in the mouth ofwhich lies the lens (Fig. 254). The cup is not perfect,however, since the invaginati
The development of the human body; a manual of human embryology . n with theectoderm of the side of the head, and this, over the areaof contact, becomes thickened and then depressed to formthe beginning of the future lens (Fig. 252). As the result of the depression of the lens ectoderm, theouter wall of the optic bulb becomes pushed inward to-ward the inner wall, and this invagination continuing untilthe two walls come into contact, the bulb is transformedinto a double-walled cup, the optic cup, in the mouth ofwhich lies the lens (Fig. 254). The cup is not perfect,however, since the invagination affects not only the opticbulb, but also extends inward on the posterior surface ofthe stalk, forming upon this a longitudinal groove and pro-ducing a defect of the ventral wall of the cup, known asthe chorioidal fissure (Fig. 253). The groove and fissurebecome occupied by mesodermal tissue, and in this, at THE EYE. 477 about the fifth week, a blood-vessel develops which tra-verses the cavity of the cup to reach the lens and is knownas the arteria 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
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
