Abstract
introduction The secretions of the thyroid gland tend to accelerate skeletal ageing while lack of the secretions tends to retard this ageing (Silberberg and Silbergerg, 1943, and others). The time of appearance of secondary ossification centers, a stage of skeletal ageing, seems to agree with this concept, for such centers are accelerated in their appearance in rats injected with thyroxin (Noback, Barnett and Kupperman, 1949) and delayed in their appearance in thiouracilinjected rats (Noback, Barnett and Kupperman, 1949), in human cretins (Means, 1937; Caffey, 1945; Albright, 1947; and others), and in thyroidectomized rats (Scow, Becks, Simpson, Asling and Evans 1948). The purpose of this paper is to consider the relation of the thyroid gland to the time of appearance of primary ossification centers of membrane and cartilage bones (which only appear during prenatal life) in rat fetuses.
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