. Human embryology and morphology. Embryology, Human; Morphology. 180 HUMAN EMBRYOLOGY AND MORPHOLOGY. wall is composed of a single layer of epithelium. The pars ciliaris retinae retains this form. The cells of the sensitive retina elongate, but the process of formation of rods and cones and other structures in the retina has not been fully followed. If Gaskell is right then the matter is simple. He believes that the epithelial cells of the inner layer of the optic cup become transformed into the fibres of Miiller; the rods, cones, and gang- lionic cells being derived from the cells which grow
. Human embryology and morphology. Embryology, Human; Morphology. 180 HUMAN EMBRYOLOGY AND MORPHOLOGY. wall is composed of a single layer of epithelium. The pars ciliaris retinae retains this form. The cells of the sensitive retina elongate, but the process of formation of rods and cones and other structures in the retina has not been fully followed. If Gaskell is right then the matter is simple. He believes that the epithelial cells of the inner layer of the optic cup become transformed into the fibres of Miiller; the rods, cones, and gang- lionic cells being derived from the cells which grow into the optic vesicle from the epiblast. The ganglionic cells, however, are more probably derivatives of the neuroblasts of the optic vesicle. At any rate an inner layer of ganglionic cells is formed which give off the optic fibres as processes. These fibres converge at the stalk of the vesicle, thus forming the optic disc; they grow inwards on the stalk, which becomes the optic nerve; some at least, perhaps all, cross in the floor of the 3rd ventricle forming the chiasma, and pass round, as the optic tracts, to ganglia situated on the mid-brain. There are also efferent fibres in the optic nerve, which end round the ganglion cells of the retina. The inner ganglionic cells of the retina probably correspond to the cells of a posterior root ganglion. According to Gaskell the optic vesicles arose in the ancestry of the verte- brates as diverticula of their alimentary canal; when the alimentary function of the canal was lost and it became neural, these diverti- cula became the optic vesicles. Tie choroidal Fissure.—Occasionally congenital fissures are seen hyaloid / ^^^\sS^^^mar9in °f Pupil optic stalk ^^ choroidal fissure Fin. 147.—The Optic Stalk and Cut), viewed on the lower and lateral aspect, showing the closure of the Choroidul Fissure. in the lower and outer segment of the iris (coloboma iridis). A white line, due to absence of pigment, may be seen in the corre-.
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Keywords: ., bookcentury1900, bookdecade1900, booksubjectembryol, bookyear1902
