Quarterly Journal of Microscopical Science, Vol s3-105, 13-20, Copyright © 1964 by Company of Biologists

Studies on the Receptors in the Cerebral Vesicle of the Ascidian Tadpole, 2. The Ocellus

¹ Department of Anatomy, University College, Gower St., London, W.C. 1

Electron microscope observations of the photoreceptor of the tadpole larva of Ciona intestinalis show the ocellus to consist of about 10 cells. It is made up of 3 parts: a pigment cup, a lens system, and a retina. A single cup-shaped cell filled with membrane-bound pigment granules lies between the lens and the retinal cells. Part of the lens cell is contained within its concavity. The pigment granules are arranged to keep stray light from falling upon the photoreceptor endings.

The lens system has 3 lens vesicles arranged in a line along the main axis of the pigment cup cell. The spherical vesicles are usually contained within a single cell. Each vesicle is separated from the rest of the lens cell cytoplasm by a single-layered sphere of mitochondria. No membrane separates the vesicle contents from the rest of the lens cell cytoplasm.

The retinal cells lie above the dorsal arm of the pigment cup cell, and have tubular processes that pass through it to end as piles of membranes closely applied to the lens cell. Morphologically the photoreceptor units of the ascidian tadpole closely resemble the visual cells of the vertebrates, the piled photoreceptor membrane corresponding to the outer segment of the vertebrate rod cell and the tubular part to the connecting piece of a retinal rod.