Quarterly Journal of Microscopical Science, Vol s2-86, 1-51, Copyright © 1945 by Company of Biologists

Memoirs: On the Eyes and Reactions to Light of Proteus Anguinus

¹ Department of Zoology, University College of the South West of England, Exeter

The structure of the eye has been reinvestigated. Development is arrested at the secondary optic vesicle stage, though growth continues for some time afterwards. The lens does not complete its differentiation; it usually disappears in adult animals, and is replaced by a proliferation of epithelial cells from the iris margin. If the lens persists this proliferation is inhibited. These facts are explicable on the assumption that the normal potencies of the iris margin are realized when the inhibitory action of the lens is removed, but, in Proteus, the typical dominance of the dorsal region of the margin is lost, so that the organization of the proliferated cells into a morphogenetic field does not take place.

The visual cells of the retina stop their development at about the time when rod and cone primordia first become distinguishable.

The optic nerve, greatly reduced, persists throughout life.

The vascular supply to the choroid is maintained through a minute vessel representing the ophthalmic artery.

The relations of the nerve and artery to the trabecula cranii are those of a typical urodele embryo and, in this respect, development in Proteus is arrested at a slightly earlier stage than in Necturus.

The development of the extrinsic eye muscles is arrested before their adult arrangement is achieved and before the M. rectus internus appears.

The skin above the eye is unmodified and the accessory cornea of Schlampp does not exist.

It is argued that the condition of the eye and some other structures in the orbit is just as much the effect of neoteny as such typically embryonic features as the skin and gills. The general tendency to neoteny was present when Proteus first adopted a cavernicolous life and was able to extend to the organs of vision when their function became unnecessary.

Proteus is sensitive to stimulation by light and responds to it by movement. The eyes are unnecessary for light perception.

Stimulation of the body and tail at any intensity and of the head at low intensities produces slight movements of adjustment and, if repeated, may lead to movements of translation.

Stimulation of the head at high intensities leads to a violent and sudden turning response after a latent period of reaction, dependent, within limits, on the intensity of stimulation.

The active factor of the stimulus is intensity, not direction.

The reactions at high and low intensities and the types of orientations they produce (‘photophobotaxis’ and apparent ‘photokinesis’) are possibly not fundamentally distinct. The subjective impression of movement at random below the threshold for shock reactions is erroneous; the movements are controlled, but the basis of control has not been discovered.

The light responses are not adaptations developed in relation to life in caves, in which they are never used. They are survivals from a period of epigean existence, and may have played a part in the colonization of caves by Proteus.