Evolutionary Aspects

The profundity of evolutionary alterations in the structure of this organ deserves some comment. In lower forms, the pineal organ functions as a light receptor and in this definition is often referred to as the "third vertebrate eye." In 1959, it was recognized that the ultrastructure of the receptor cells in the parietal eye of the lizard resembles that of retinal cones, and the most highly differentiated sensory elements of this type occur in the pineal vesicle of the lamprey, the frontal organ of the frog, and the parietal eye of the lizard. The similarities between the neuronal organiza-

Hormones, Second Edition

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