The thyroid gland of the spurdog, Squalus acanthias L., has been studied, and its growth and seasonal variations in activity described. Thyroid weight was linearly related to body weight in both sexes. There was some evidence of a period of accelerated growth of the gland at the time of attainment of sexual maturity in the females, but not in male fish. Follicular growth bore a linear relationship to thyroid growth in both sexes. This relationship was marked in the males, but in females there was a great variability in the size of the follicles. Follicular cell height also showed a linear relationship to thyroid weight in female fish but not in males. The significance of these findings, and of the sex differences in thyroid growth found in this species, are discussed in relation to other observations on the growth of the thyroid gland in fish.

Seasonal variations in thyroid activity in the spurdog were studied. Female spurdogs are ovo-viviparous, gestation lasting two years, whilst migration occurs annually; these two events could therefore be considered independently in relation to changes in thyroid activity. During the summer months, the thyroid of the adult female was slightly active, and was composed of regular follicles containing abundant homogenous colloid. There was a marked fall in thyroid weight each winter, when the gland became active and colloid was rapidly withdrawn. Finally the thyroid became structurally disorganized and almost entirely depleted of colloid reserves. Recovery occurred slowly but the thyroid had re-entered the resting condition in spring. These changes were found in the thyroids of females in both years of pregnancy, suggesting that thyroid activity was not primarily associated with reproductive events. A consideration of the migratory movements of the adult female spurdogs showed that these coincided with periods of thyroid activity, and it is suggested that migration and thyroid activity are closely related.

Seasonal changes in thyroid activity were found in adult male spurdogs. The gland was inactive during the summer months, but became active each winter. These variations were less marked than those seen in the adult females; there was little evidence of the structural disorganization and almost total withdrawal of colloid which was typical of the thyroid of the adult female spurdog in winter. Again, the annual changes in thyroid activity corresponded with the period of annual migration, suggesting that the two are related. The differences in the thyroid activity of the sexes are believed to be associated with their markedly different reproductive cycles. Juvenile spurdogs of both sexes, which do not make regular seasonal migrations, showed no marked seasonal fluctuations in thyroid activity; this provides further evidence of the close link between thyroid activity and migration in the spurdog.

The development of the thyroid gland in the intra-uterine embryo is described. In the smallest embryos (2 cm in length) the thyroid consisted of a simple aggregate of epithelial cells; primitive thyroid follicles were first seen in embryos of 8 cm. Follicular differentiation and growth were continuous throughout intra-uterine life, and were accompanied by a progressive accumulation of colloid; there was no evidence of an abrupt structural or functional differentiation of the thyroid at any stage during gestation.

Conspicuous seasonal changes were found in the activity of the thyroid gland of the intra-uterine embryos. During the winter months, in both years of gestation, the foetal thyroid became depleted of colloid reserves and structurally disorganized. These changes occurred in small embryos during their first year in utero but were more readily seen in the larger embryos where the thyroid had become well differentiated. The recovery of the embryonic thyroid was rapid, and the gland was reorganized in both near-term and newborn young. The changes in the embryonic thyroid closely resembled those seen in the thyroid of the pregnant female. The significance of this is discussed in relation to the existence of a functional link between maternal and foetal thyroid cycles.