General Summary1. The plumage of the Brown Leghorn fowl is analysed and the extent of its dependence on the activity of the thyroid and gonad is discussed.2. Hypothyroidism results (a) in a diminution in the amount of melanin and a coincident increase in the red pigment; (b) in an increase in the amount of fringing due to a lack of barbule formation. The female pattern tends to disappear.3. Hyperthyroidism in the male gives exactly the opposite effect—i.e. the melanin increases in amount while the red pigment and fringing tend to disappear. In the female the effect of hyperthyroidism is slight, and with moderate doses of thyroid there is practically no modification of the female pattern of pencilling and no melanin appears in the breast. The neck hackle, however, becomes darker and shows a decrease in fringing.4. From these results, together with what is known concerning the effect of gonad on plumage, a hypothesis is advanced, that whereas the plumage typical of the male is developed independently of the gonad and depends for its maintenance on a certain level of thyroid functioning, both gonad and thyroid play a part in regard to that of the female; the former stimulates the latter to a higher level of activity than that present in the male and so indirectly causes a hyperthyroid effect on the feathers. At the same time it modifies this condition by acting directly on the feathers and restricting the deposition of melanin.5. The fact that in both sexes the plumage in the young chick is similar to that of the female puts this theory in question. There is evidence to show, however, that yolk may have a modifying effect on plumage similar to that of the ovary, and thus the doubt regarding the validity of the hypothesis can be removed by the suggestion that the chick plumage develops under the influence of the yolk in its own yolk sac. This second theory gains confirmation by a study of the atypical plumage which is occasionally met with in growing females, and also by a consideration of the type of plumage developed in juvenile females following the successful implantation of testis.

1. Peculiarities of form and function in the accessory sexual apparatus of a number of experimental pullets, derived from eggs injected with œstrone at an early stage of incubation, are described and discussed. 2. Only one of a group of 6 females laid normal eggs when sexually mature, the remainder producing eggs devoid of shell or lacking both shell and egg membranes. This phenomenon is ascribed to an abnormality of the oviduct. 3. All the experimental birds autopsied possessed two oviducts which showed incomplete development both as regards total length and length of component parts. 4. The total weight of the two ducts approximated that of the single left oviduct in the normal hen. 5. It is suggested that either the œstrone injection to the egg interfered with the normal processes of differentiation of the mullerian duct into the sex duct in the embryo, rendering it incapable of efficient response to the hormone stimulus subsequently supplied by the functional ovary of the individual, or that the normal level of secretion of female hormone by the fowl's ovary is sufficient only for the complete development of a single duct.

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During the winter of 1937–38 it became necessary to obtain the maximum number of offspring from a certain pen mating for the purposes of an experiment unconnected with the present report. Accordingly all suitable eggs were incubated, and since chicks were required all the year round the data which accumulated related to periods extending beyond the normal hatching season. Not long after the beginning of the project strikingly consistent differences between the records of the individual females became apparent, and interest in this phenomenon led to the decision to incubate every intact egg from the pen, irrespective of any normally undesirable characteristics of shell texture or shape. This was done for two years (1938–39), and provided the main data for the study of fertility and embryonic mortality reported here.