Adult white-footed mice,Peromyscus leucopus, were kept either in long photoperiod (LP; LD 16:8) or in short photoperiod (SP; LD 8∶16) for six weeks. Ovaries were serially sectioned and examined histologically. Preantral follicles were observed to release their oocyte into the ovarian stroma. This phenomenon is termed “preantral intra-ovarian oocyte release (PIOR)”. The first step in PIOR appeared to be the interruption of the basal membrane and the thecal layer. PIOR was considered incomplete when the oocyte was retained in the follicular compartment. PIOR was complete when granulosa cells were seen to be herniated and the oocyte released either into the interstitial cortical tissue or into a corpus luteum. During incomplete PIOR signs of atresia were increased, whereas during complete PIOR they were decreased. Resumption of meiosis was not seen. PIOR was observed in 19.5 of the ovaries after LP and in 18.8 of the ovaries after SP. PIOR is regarded as a sign of follicular at
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