1. Illumination of the dorsal region of the head of an ammocoete larva is followed by movements of the animal, but only after exposure for longer periods than are necessary to elicit responses from the tail.

2. Since this reaction persists unaffected after removal of the pineal and paired eyes, it is concluded that it is produced by the direct effect of light on some tissue in the brain.

3. Larval and adult L. planeri show very pronounced daily rhythms of colour change, becoming pale at night and dark during the daytime.

4. Continuous artificial illumination of the animals produces maximal darkening and stops the diurnal rhythm.

5. When animals are left in total darkness the diurnal changes usually persist, though diminished in extent; sometimes the melanophores come to rest in the expanded phase.

6. Since section or faradic stimulation of spinal nerves is not followed by local changes in the melanophores, it is concluded that these are not under nervous control.

7. After removal of either the whole pituitary complex or its pars nervosa and intermedia the animals become maximally pale, and remain so indefinitely in spite of changes of illumination.

8. Injection of extracts of mammalian posterior pituitary lobe causes darkening of such hypophysectomised lampreys.

9. Pituitrin was also found to be capable of maintaining the expansion of isolated melanophores.

10. After removal of the pineal complex from ammocoetes the rhythms of colour change were interrupted, the melanophores remaining in the expanded phase under all conditions of illumination. Removal of the pineal of adult L. planeri disturbed the colour rhythm, which was then completely abolished if the paired eyes were also removed.

11. Thus the paling of an ammocoete when it passes from light to darkness is probably due to the inhibition of posterior pituitary secretion by nervous impulses set up by the change of illumination of the pineal complex.

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