. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Evolution. 38 BLIND VERTEBRATES AND THEIR EYES. near the eye. In the adult the sclera is a layer of uniform thickness except in the region of the entrance of the optic nerve. It is not usually separated from the adjoining parts of the eye, but in places is retracted a short distance from the choroid coat by the action of reagents. It is for the most part fibrous, with few compressed nuclei, and varies from i8/x to 40/j, in thickness. In the larva a narrow cartilaginous band surrounds all but the ventral wall of the
. Cave vertebrates of America; a study in degenerative evolution. Cave animals; Evolution. 38 BLIND VERTEBRATES AND THEIR EYES. near the eye. In the adult the sclera is a layer of uniform thickness except in the region of the entrance of the optic nerve. It is not usually separated from the adjoining parts of the eye, but in places is retracted a short distance from the choroid coat by the action of reagents. It is for the most part fibrous, with few compressed nuclei, and varies from i8/x to 40/j, in thickness. In the larva a narrow cartilaginous band surrounds all but the ventral wall of the eye. In a specimen 35 mm. long the width of the band is about 30 /a, its thickness 16 /x. In three adult specimens the sclera of only one had any traces of cartilage. In the right eye of the adult specimen 103 mm. long a cartilage about 36/u. thick, 60 ju, wide, and not more than 40/n long is found on the upper face of the eye. The absence of this cartilage in the adult has probably no connection with the degeneration of the eye. Its presence is probably a larval characteristic which disappears as the gills disappear during the Fig. 10. (a) Section of Rctin,a, exclusive of Pigment Cells, of Larva 3 •; mm. long, (b) Tangential Section through Rods and Cones about on Level with Innermost Extent of Pigment (seen on Right) showing Relative Sizes and Abundance uf Rods and Cones, (c) Section of Ret- ina of Larva 48 mm. long, (d) Section of Retina of Larva 00 mm. lone, d) Tangential Section showing Rods and Cones at about Inner Limit of Pigment (seen on Left). ( f) Section of Retina of Adult 106 mm. long. (,g) Tangential Section at about Inner Limit of Pigment. (It) Section of Retina of Adult 97 mm. long. The average thickness of the cornea is 40/x. In the adult it is covered by a layer of stratified epithelium, 25 /u, in thickness, consisting of three rows of cells. The cells of the inner row are columnar in shape, those of the middle row rounded, and those of the
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