Abstract
1. In the normal course of the postnatal development of lymphoid organs in the rat, a remarkable transient elevation of the average number of mitochodria was observed in circulating lymphocytes, during the period between 4 weeks and 2 months after birth.
2. This period coincides with the period during which the processes of new formation and differentiation of Flemming's secondary nodules proceed in the mesenteric lymph nodes and splenic white pulp, the former in particular. After maturation of Flem - ming's nodules in these organs had been accomplished, the mitochondrial number in circulating lymphocytes considerably fell and did not rise agin.
3. These findings agree fairly well with the previous observations by Osogoeet al. (1953) and A way a (1956) on rabbits and provide further evidence to indicate that a marked rise in mitochondrial number in blood lymphocytes takes place as a consequence of extensive new formation of Flemmin g's secondary nodules in the lymphoid organs, which, in turn, indicates an increase in production and delivery of lymphocytes from these organs to the blood.
