Abstract
Summary: 1. Sexual periodicity in many animals is largely conditioned by variations in the daily light period. That egg production in hens may be controlled by the use of artificial lights has long been known.2. The action of light is probably connected with the activity of the anterior lobe of the pituitary.3. The natural variation in daily light period at different latitudes has been used to study the effect of light on egg production throughout the world. Curves have been constructed of egg production grouped in 5° of latitude and the correlation with the hours of daylight at those latitudes studied.

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