ONE FlGIURE
Since the pioneer work of Long and Evans ('22) much emphasis has been placed upon the size of corpora lutea in rats under various conditions. The most striking observation has been that during the latter half of pregnancy the corpora lutea are larger than at any other time. Long and Evans distinguished between four kinds of corpora lutea found in the rat ovary at different times, viz., those of ovulation, pseudopregnancy, pregnancy and lactation. They pointed out that the maximum diameter of the corpora lutea of ovulation is 1.2 mm.; of pseudopregnancy, 1.2 to 1.4 mm.; of pregnancy, about 2.0 mm. and of lactation, 1.2 mm. Swezy ('33) reported a few measure:ments of corpora lutea which, for the most part, agree with t,hose of Long and Evans. She pointed out, as did the latter investigators, that the corpora lutea of pregnancy cannot be distinguished from those of ovulation until approximately the tenth day of gestation. Between the tenth and fourteenth days the corpora lutea grow to the maximum diameter of about 2.0 mm., and remain at this size until the end of pregnancy. Her measurements