Pattern Analysis in Plumage. IV. Order of Asymmetry in the Breast Tracts

In experiments with female hormone in this laboratory it was found that when the mark due to the hormone was restricted to one vane-half in the definitive breast feather, other feathers were occasionally found in the same tract bearing a similar mark on the opposite vane-half. There are nine feathers in a transverse row across each breast tract; counting laterally from ventral mid-line of the bird, we shall designate the feathers of such a row by the numbers one to 9. Ordering the various feathers in which the asymmetrical mark was found according to this notation, it could be shown that feathers one to 5 bore the female hormone mark predominantly on one vane-half; feathers 7, 8 and 9 carried the mark predominantly on the opposite vane-half. Feather number 6 in this composite row showed little if any asymmetry in distribution of the female hormone mark, i. e., both vane-halves were marked to about the same degree. Finally, the degree of asymmetry increased from feather to feather in the order of distance from number 6: the greatest asymmetry of marking thus occurred in those feathers nearest the ventral and lateral margins of the breast tract, but of course on opposite vane-halves.

These observations pointed to a true reversal of asymmetry within each breast tract with respect to a secondary antero-posterior axis of symmetry. The position of the secondary axis was apparently given by the sixth feather of each transverse row. The precision of response occasionally observed indicated, moreover, that the relations of asymmetry of feathers ventrally and laterally from the assumed secondary axis of symmetry were of an exact quantitative order.