THE VISUAL CELLS OF THE PLATYPUS (Ornithorhyncus)

Abstract

IN all vertebrate animals below the mammals it is the rule to find the cones present in both single and double forms. There are two varieties of the latter-" twin " and " double " cones. The members of the former are alike in every detail, whilst in the latter they differ in structure. The chief cone resembles its solitary neighbours and contains an oil-droplet, if they possess one; the accessory stains more delicately and contains no oil-droplet. In both types of double cones, although the elements appear to be fused in the greater part of their length, the end-plates and distal segments are free, and both units probably function independently. Because of the delicate structure of the accessory cone and of its intimate association with its partner, its observation is not always easy.