On the change over in the oestrous cycle in animals after transference across the equator, with further observations on the incidence of the breeding seasons and the factors controlling sexual periodicity

Abstract

In the Croonian Lecture (1936) I adopted the view that sexual periodicity is primarily a function of the gonads which show an alternation between a condition of activity and one of rest, but that the reproductive rhythm, while metabolically conditioned and to some extent- directly controlled by the general environment, is in the higher forms of life regulated further by exteroceptive stimuli conveyed by nervous paths to the hypothalamus and thence transmitted to the anterior pituitary. The evidence for this view is there summarized, and it becomes apparent that in mammals and birds at any rate the nervous system plays an important part in the regulation of the oestrous cycle. In a large number of animals the incidence of daylight is almost certainly an important factor in controlling the cycle. The ruminants, however, are exceptional since if they react to light at all it must be to diminution rather than to increase. That they react to seasonal change in a remarkable way is shown especially by those natural experiments in which individual animals of varieties or species that normally breed once annually, have crossed the equator and as a consequence have been induced to have two sexual seasons in one year. At any rate these cases cannot be explained simply as due to an endocrine rhythm.