Ovaries from 16to 17-day-old mice were cultured for 4 days in the presence of various concentrations of highly purified FSH and LH. Serial sections were prepared and parameters of all follicles with four or more layers of granulosa cells were measured. FSH significantly increased the total number of such follicles and in particular the number of small ones. It did not significantly increase the number of large ones although it markedly augmented the number of follicles with small antra. LH

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... ged the mean follicle diameter by multiplying the number of large follicles while having no effect on the number of small ones, but it did not significantly increase the number of follicles with antra.