The physiological mechanism of the gonad-stimulant effect of light rays on the pre-pubertal duck testis, is the basis of any chronobiolgical study of this effect. 2. The light stimulates the chain formed by the retina, the optic nerve, the hypothalamus and the anterior part of the pituitary. This chain includes 2 photo-receptors, a superficial one, the retina and a deep one, the hypothalamus. The retina is here only sensitive to orange and red rays. Consequently an autonomic retina intervenes instead of the visual retina. The hypothalamus, on the other hand, is sensitive to all visible rays when they are brought directly in contact with it through a quartz rod placed in the orbit which has been previously emptied. 3. The hypothalamus, which is thus enlightened directly is more than a hundred times more sensitive than the retina. This great sensitivity allows normal stimulation by the most penetrating rays in the orbital region. 4. Ultra-violet and infra-red rays are inactive on the autonomic retina and on the hypothalamus. 5. The neuro-secretory cells of the latter, stimulated directly or indirectly, secrete hormones which via the axones reach a capillary network covering the median eminence, then, by series of portal veins, the anterior part of the pituitary, the corresponding cells of which secrete various hormones, which through the systemic circulation reach the corresponding target organs and in particular the gonads. It is thus a neuro-hormonal mechanism which the external factor light, brings into play to stimulate various functions of the autonomic life of the organism.
