Pattern Analysis in Plumage. ∗ I. Curve of Barb Growth

Abstract

Lillie and Juhn 1 reached the conclusion that “general physiological conditions produce morphological diversity in genetically identical feather germs, and . . . . the main cause of the diversity of action is definitely oriented, accurately regulated differences of growth rate within the germ.” (p. 177.) It was further pointed out that, since pattern is primarily differentiation in respect of barb level, the curve of barb growth rate becomes highly important in the interpretation of pattern. A curve given in the earlier study was believed to describe the growth of a barb from its origin at the ventral collar limit to its emergence on the rhachis. From this curve of barb growth it was concluded that the rate of barb growth decreased continuously from a maximum value at the point of origin to a minimum value at the dorsal limit of the collar. Differentials in growth rate are greatest through the ventral-most region of the collar and decrease progressively to the dorsal limit of the collar. Growth rate through the ventral-most quarter of the collar is, in terms of the curve of barb growth given, several times the rate of growth through the 2 dorsal fourths of the collar.

Further study of the subject has confirmed the idea of a growth gradient in the barbs, but at the same time has led to the conclusion that the gradient was originally represented as too steep from ventral to dorsal regions. Several lines of evidence bearing on this conclusion are as follows:

1. Completely limited pattern reactions. Lillie and Juhn have shown that it is possible to induce the female pattern completely across one vane-half of wing covert feathers without visible reaction in the opposite vane-half. This complete asymmetry is interpreted in terms of differential growth rate levels of barbs on the 2 sides of the germ. It follows that there can have been no regions of equal growth rates in the 2 collar halves during the period of pattern determination. The maximum rate of barb growth in the vane-half reacting to female hormone must accordingly have been less than the minimum growth rate of the vane-half in which the reaction does not appear.