Abstract

The cells of Hesse in the ventral wall of the neural tube of Amphioxus, long assumed to be photoreceptors, were found by electron microscopy to possess many narrow irregular tubules, or diverticula of the cell membrane, immediately adjacent to the melanocyte associated with each receptor cell. Cilium-like structures are absent. Accordingly, Hesse cells do not resemble vertebrate photoreceptors. The ependymal cells in the posterior floor of the cerebral vesicle (infundibular organ?) have cilia and long filaments, both containing strings of vesicles. These vesicles may be suggestive of the evolutionary forerunners of the flattened sacs or discs in the outer segments of vertebrate photoreceptors.

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