The choroidal tapetum lucidum of Latimeria chalumnae

The choroidal tapetum of Latimeria chalumnae has been examined by optical, and by transmission and scanning electron microscopy. The retinal epithelium, which contains no pigment, is separated from the tapetum by Bruch’s membrane and the well-developed choriocapillaris. The tapetum itself consists of cells containing stacks of flat hexagonal crystals, shown by paper chromatography to be guanine. The crystals are arranged parallel within each cell; most are also parallel to those in neighbouring cells, though the crystals in some cells make a large angle with the general crystal plane. In the main the crystals lie so that their plane is approximately perpendicular to the optic axis of the eye. Owing to the retinal curvature, the angle of the crystal plane varies with position in the eye. The conditions for reflexion by constructive interference are fulfilled. Few choroidal melanocytes lie among the cells of the tapetum, which is not occlusible.