STUDIES ON THE PHYSIOLOGY OF REPRODUCTION IN BIRDS

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Abstract
The bursa Fabricii attains its maximum size at the age of 2.9 (common pigeons) and 3.1 months (ring doves); age being calculated from the beginning of embryonic development. Its period of rapid growth coincides with rapid growth of body and of thymus, and with very slow growth in the gonads. The involution of the bursa begins, at least in early maturing birds, immediately after its maximum size is attained. At precisely this same time the thymus begins its partial involution, and testis and ovary begin a greatly increased rate of growth. Bursa involution is usually, but not invariably, practically complete at the time sexual maturity is attained. The more intimate knowledge of the bursa emphasizes its relationship to a period of extremely rapid body growth, to a period of repressed growth in the gonads, and to the time of attainment of the capacity of either sex to reproduce. These data provide some evidence that this organ has an endocrine function. Also, that the bursa is most closely associated with the thymus, and that these 2 organs may have an identical function. Testis and ovary grow very slowly during the period of rapid growth in the bursa, thymus and body; this slow gonad growth in the early postnatal period is interpreted as modified or suppressed growth associated with active growth in the bursa and thymus. The rate of body growth in doves and pigeons is exceptionally high as compared with that of any other bird, mammal, or other vertebrate hitherto studied. Birds alone of these groups possess a bursa Fabricii. Data are given for the age at which sexual maturity is attained in ring doves and in common pigeons. An unexpected disparity of size is found in the testes and ovaries at hatching[long dash]the ovary being about twice as large as the testis, although the adult testis is 3 to 5 times as large as the adult (resting) ovary. This is interpreted as a result of the presence of the female or follicular hormone in the egg, and of its specific action upon the embryonic gonads. This modification of the embryonic growth of organs as the result of the regular, though essentially accidental, inclusion of a maternal hormone within the egg is of interest in the theory of development or heredity. The conditions actually found are currently considered normal, though they are quite as abnormal as those effected by the experimentalist. In development, the word "normal" can merely mean "the usual.".

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