Preliminary observations on the development of photoreceptor cells in the spade foot frog Rana breviceps (Schneider)

Abstract

Serial sections of the eyes of tadpoles and froglets of Rana breviceps of various developmental stages between stags 26 (Hatching) and 55 (end of metamorphosis) were examined. At stage 26 different layers of sensory retina are not yet segregated. Differentiation of visual cells begins by stage 30 when small cytoplasmic processes of visual cells are seen projecting through the outer limiting membrane. At this stage the cytoplasmic process of every visual cell contained a cavity which did not stain. By stage 31 three types of visual cells can be recognized : red rods, single cones and accessary member of double cones. Single cones have oil droplets. Paraboloid is present in some single rods and in the accessary members of double cones. The chief cone is not yet associated with the presumptive double cones. By stage 32 the double cones consist of both chief and accessary members. The paraboloid is now present only in the accessary members of double cones. Green rods are first seen in the mature tadpoles of stage 45/46. At stage 32, red rod : single cone ratio was almost 1 : 1 and at stage 39/40 it was 5 : 1 At stage 45/46 single cones were rare in the ventral retina but relatively more in the dorsal retina. Number of double cones gradually increased with approach to metamorphosis. Red rods were predominant type in froglet retina with fewer cones, double cones and green rods in this order.