SEASONAL CYCLICAL CHANGES IN THE THYMUS OF THE MALLARD*

In the course of an investigation into seasonal changes in the endocrine organs of wild mallards (Anasplatyrhinchas p. (L.)), a striking seasonal cycle was found in the thymus affecting both the amount of thymus tissue and its histological structure. This appears to be the first description of changes of this nature affecting the thymus apart from a statement by Jolly (1913) in connexion with age involution of the thymus in fowls, to the effect that this tissue is much influenced by seasonal and nutritive changes. Aime (1912) has described a seasonal regeneration of the thymus in turtles, but it will be shown below that this must be regarded as a different type of response from that found in the mallard. MATERIAL AND METHODS Male and female adult mallards were shot in one of the London Royal Parks at fortnightly intervals for the period May to July 1944. A second series of birds was obtained from October 1944 to January 1946. During the period November to December 1945 six adult males were killed within a period of 4 weeks in order to give an indication of the degree of individual variation in thymus weights. One male and five female immature birds were also obtained, while in July and August 1946 three additional adults were shot to amplify the data for the period of maximal thymic regeneration. The freshly dissected lobes of thymus were weighed on a torsion balance when weighing up to 500 mg., or if heavier than this on a chemical balance. The glands were then fixed in formol acetic Muller's solution, and serial sections of part of a lobe stained in Delafield's haematoxylin and alcoholic eosin. RESULTS The avian thymus develops from the third and fourth branchial pouches, and in the bird just hatched appears as a long thin formation running from the level of the hyoid arch cranially to that of the eleventh segment caudally, on both sides. Where this mass is crossed by the cervical nerves it becomes constricted and finally divided into several discrete lobes (Streseman, 1934). * This paper is part of a thesis for which the degree of Master of Science in Physiology has bee