Observations on physiological factors influencing the genetic coloration of fowl plumage.

1. Evidence is given that there is in the Brown Leghorn Fowl (light brown variety) a direct correlation between the rate of growth of the feathers and the amount of black pigment deposited in them. 2. The effect of hormones (male, female, thyroid and anterior pituitary extracts) on growth rates and on pigmentation is considered. 3. Tests are described to discover whether female hormone has an effect on the production of melanin by the tyrosine - tyrosinase reaction. 4. A suggestion is made regarding the previously known fact that there are in urine unidentified substances affecting (inhibiting) the tyrosine - tyrosinase reaction.