. The biology of the amphibia. Amphibians. 332 THE BIOLOGY OF THE AMPHIBIA tive to light, the latent period of reaction was more than doubled by covering the vestigial eyes with lamp black. On the other hand, Obreshkove (1921) concluded that the well-developed eyes in the tadpoles of Rana clamitans played no part in the responses of the tadpoles to light. Probably under other con- ditions of illumination the eyes would have important functions. Rods and cones are evenly distributed over the retina of Nec- turus (Palmer, 1912), but in Rana a thickened portion of the retina seems to mark a regio
. The biology of the amphibia. Amphibians. 332 THE BIOLOGY OF THE AMPHIBIA tive to light, the latent period of reaction was more than doubled by covering the vestigial eyes with lamp black. On the other hand, Obreshkove (1921) concluded that the well-developed eyes in the tadpoles of Rana clamitans played no part in the responses of the tadpoles to light. Probably under other con- ditions of illumination the eyes would have important functions. Rods and cones are evenly distributed over the retina of Nec- turus (Palmer, 1912), but in Rana a thickened portion of the retina seems to mark a region of acute vision since cones are especially abundant here. Chievitz (1889) reports two species of Bufo as having a shallow depression in this region, an incipient. Fig. 120.—Visual cells of a salamander. The rods and cones from the retina of a larval Ambystoma. , paraboloid; , cone nucleus; , double cone; EL, ellipsoid; , outer segment of cone; , outer segment of rod; „ rod nucleus. (After Detwiler and Laurens.) fovea developed in species known to have a better vision than Rana. Keenness of vision is dependent not only on a focusing of the lens in such a way that a clear image is thrown on the retina but also on proper illumination of the retina. In strong light the iris rapidly contracts and the pigment cells in the retina send out processes which cut down the amount of light reaching the rods and cones. The dark-adapted eye of the frog exhibits not only a contraction of the pigment cells but a rod contraction and a cone elongation. The pigment contraction is, however, influenced by temperature. Less contraction is found to accompany exposures at high temperatures than at low temperatures in the dark (Detwiler and Lewis, 1926).. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original work
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Keywords: ., bookcentury1900, bookpublishernewyorkmcgr, booksubjectamphibians
