A Study of Retinal Development in Larval Herring and Sole

It is known from earlier work (Blaxter & Jones, 1967; Blaxter, 1968 a ; Blaxter & Staines, 1970) that the larvae of many species of teleost have a pure-cone retina. The rods only appear well after hatching and often as late as metamorphosis. This means that much of the larval life may be spent without fully developed eyes. The rods are difficult to resolve under the light microscope using paraffin sections (about 5 µm) and it is therefore easier to compare the number of cones with the prominent visual cell nuclei in the outer layer (ONL) in such preparations. The ratio has been found to be 1:1 in the early stages and changes as the rods develop, reaching ratios of 1:15 or more in adult fish. Thus, for every cone there may be 14 or more rods.