Previous experimental work concerning enlargement of the thymus has dealt mainly with the regeneration which follows partial or subtotal removal, or with the regeneration which follows transplantation. We planned our experiments to obtain evidence of uncomplicated hypertrophy by removing one lobe and studying the enlargement of the remaining lobe. Our results clearly indicate that such hypertrophy takes place in young animals. They emphasize the functional importance of this gland before puberty, since the compensatory enlargement does not take place after puberty. The results further indicate that with the onset of involution there is a marked or almost total decline in the function of the gland, in spite of the fact that anatomically the thymus may persist to old age. With the appearance of a stimulus for regeneration, as, for instance, suprarenalectomy, the involuted gland takes on functional activity.

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