Further experiments with thyroidectomy in Amphibia

Abstract

The thyroid gland anlage was removed from a large number of Rana sylvatica larvæ 6 to 8 mm. in length. From the time the larvæ averaged 18 mm. in length (225 experimental and 182 control animals) the experimental larvæ grew more rapidly than the controls, and most of them became ultimately much larger than the controls, the largest being 72 mm. in length as against 50 mm. for the controls. Thirty thyroidless animals failed to metamorphose and in twelve others the thyroid regenerated, permitting metamorphosis at the normal time. The rest of the experimental larvæ were killed or died before this time. Six of the thyroidless larvæ are still alive, nine months after the normal time of metamorphosis. They practically ceased growing six months ago.

In the absence of the thyroid the secretory (anterior) portion of the hypophysis underwent physiological hypertrophy to compensate for the loss of the thyroid, as both normally are concerned with metabolism, although not in an exactly similar way. This hypertrophy probably accounts for the gigantism and infantilism (retention of the larval form) of the thyroidless larvz. Normally, synapsis begins in the testis after metamorphosis, and in the ovary a short time before metamorphosis. In our experimental animals so far examined, synapsis in the testis was practically completed at the time of metamorphosis of the controls and differentiation of the ovary also was hastened greatly. The hypertrophy of the hypophysis probably accounts for this sexual precocity. The growth in size of the gonads continues with that of the other parts of the body and is independent of the changes normally occurring in metamorphosis. Both processes, however, depend upon the general metabolism. Failure of metamorphosis is due to abnormal metabolism, brought about, either by loss of normal thyroid secreton or by abnormal secretion of the hypophysis which is affected by removal of the thyroid.