659female rats ranging from 3 to 6 months in age. Infected animals and animals having irregular oestrus cycles or cycles over 6 days in length were excluded.Animals of the same strain were kept under the conditions and on the diet described Vaginal smears were made at intervals of 12 hours or, in most cases, of 8 hours, and oestrus was defined as the period at which an abundance of cornified cells was obtained, after the preceding smear had shown nucleated epithelial cells. Dioestrus was defined as 48 or more hours after this time.Three animals with very regular cycles were killed 20 hours before the expected onset of the next oestrus. The animals were killed with chloroform and the pituitary, thyroid and adrenal were quickly removed and weighed in a closed weighing bottle to 0.1 mg. The data on the adrenal have been previously reported.6 The pituitary weights are thought to be accurate to about 0.2 rng. since the gland need not be dissected from surrounding tissue. The thyroid weights are much less so because of the difficulty of accurate and rapid dissection. They also include the parathyroids. The mean actual and relative weight of the pituitary and the probable error of this mean were calculated for each group oi animals. The resulting figures indicate definite and significant increase in the weight of the pituitary during oestrus, with gradual and progressive decrease during dioestrus. This decrease in animals killed 52-72 hours after oestrus amounted to approximately 20% of the relative weight at dioestrus.The thyroid showed no significant change in weight, but the wide variation in thyroid weight in the entire series is sufficient to obscure any except an extreme change.McKinley and Soule' reported the successful cultivation of Mycobuterizcnz Zepae, obtained from Puerto Rican lepers, on several 6Andersen, D. H., J . Physwl., 1932, 74, 49.
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