Abstract: | Aberrant thyroid tissue was observed at the base of the heart in seven of 2634 mice of the BALB/c strain. Such tissue was not seen in any of 1033 mice of the Strong A strain. The normal thyroid gland of the BALB/c mouse was observed to be more loose than that of the Strong A mouse and isolated follicles were frequently observed in tissue sections at either pole of the gland which were not attached to the body of the thyroid. |