Evolution.

Society for Experimental Biology (Great Britain)

Date:: 1953

Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Evolution. Source: Wellcome Collection.

413/484 (page 381)

OF THE RETINA IN VERTEBRATES 383 Here, then, in this optic Anlage we have three groups of cells which may become photosensitive, and it is tempting to believe that this photo¬ sensitivity probably depends on some common factor. Moreover, this common factor is probably linked with the common origin from this particular region of the neural tube, since the ependymal cells lining the cavity of the diencephalon of several vertebrates have recently been found to be sensitive to light, and this property is possessed by the ependymal infundibular organ of Amphioxus (Walls, 1942). This photosensitivity must necessarily depend upon the absorption of light and thus on the presence of a pigment, for a completely transparent cell would be incapable of absorbing the light. The nature of the pigment or pigments is thus a matter of extreme importance. There are at least four groups of pigments which are known to occur in the cells of the optic Anlage, and the problem is therefore to decide which, if any, is the effective one. The four groups include melanin and its precursor dopachrome (Bu'Lock, Harley Mason and Mason, 1950); pigments of the haem group; riboflavin; and various carotenoids, of which visual purple is a derivative. Of these, there is no evidence that melanin, in spite of its almost universal presence in photoreceptive structures, plays any direct part in vision among the vertebrates except the obvious one of acting as a protective screen rather like the black pigment inside a camera. It is not, so far as is at present known, photosensitive in the ordinary way, although its production may be accelerated by light; since, however, its precursor, dopachrome, is red in colour, this substance might possibly contribute indirectly to colour vision under some conditions. Moreover, since eosin has been shown to sensitize nerve fibres to light, it is perhaps unwise to dismiss melanin completely from consideration, on the grounds that it does not bleach in light. All that can be said now is that there is no evidence to suggest that it is used. Similar remarks probably apply also to the haem pigments, e.g. cytochrome, haemoglobin, etc. ; again there is no evidence that, in spite of their wide occurrence in animal cells, they have ever been used for the perception of light. Riboflavin, on the other hand, occurs in especially large quantities in the retina, is unstable to light, is fluorescent and has been considered (Galston, 1950) to be involved in the visual process, especially in connexion with night vision, although it is certainly not the prime factor in this activity. Visual purple (rhodopsin) and the related carotenoids are, on the other hand, both repeatedly and inde¬ pendently used in eyes and photosensitive structures in many different parts of the animal kingdom and even in plants and fungi. For example, carotenoids are believed to be responsible for the stimulation of growth which takes place in certain fungi [Phycomyces) in response to light, and