1. The weight of the thyroid of the normal anoestrous ferret of both sexes is approximately half that of the oestrous ferret, but there is little histological difference.

2. Hypophysectomy during oestrus or anoestrus decreases the weight of the thyroid of the ferret to a level slightly lower than that for the normal anoestrous ferret.

3. In the hibernating hedgehog the thyroid becomes hypotrophic (Adler, 1920), and a similar condition is found in the hypophysectomised animal.

4. No change in weight of the thyroid of the guinea-pig is noticed following hypophysectomy, but atrophy of the vesicular epithelium and distention of the vesicles with non-vacuolated colloid is regularly observed.

Hypophysectomy in birds leads to similar atrophic changes in the thyroid.

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