Growth Rate and Hormone Threshold with Reference to Physiology of Development

Abstract

This paper records briefly experiments described in detail elsewhere establishing a direct relationship between threshold of reaction and rates of growth in feathers of the brown Leghorn fowl. The material consists of regenerating feather germs. Graded injections of thyroxin are recorded with increasing doses from the base to the apex of the barbs in''the form of black pigmentation and extension of barbule formation, in the case of saddle and neck hackle feathers. This coincides with gradation of growth rates in the formation of the individual barbs (Lillie & Juhn). The same relationship is exhibited in the reactions to graded doses of female hormone. Regional gynandromorphism of feather tracts depends also on the relation between rates of growth and thresholds of reaction. Bilateral asymmetry in growth rates either in the individual feather -or in the entire bird are also reflected in the threshold of reaction. Another principle, viz., rate of reaction, also affects the form of experimentally determined patterns. A simple physiological mechanism of control (such as a hormone circulating in the blood) operating on a developing system, the feather germ, which possesses gradients in growth rates, can produce an astonishing variety of results in pattern, including structure. The agreement between the results described and Child''s theory of the axial gradient is emphasized.