The Phases of the Oestrous Cycle in the Adult White Rat

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Abstract
The duration of the oestrous cycle in thirty-nine adult rats derived from ten litters was 4.4 ± 0.04 days. The variance in cycle length tended to be less within than between litter-groups (P≏0.05). It was significantly less for individual rats than between individuals (P<0.001). The mean duration of early oestrus was 18 hr., oestrus 25 hr., late oestrus 5 hr., early dioestrus 24 hr., dioestrus 28 hr. and late dioestrus 7 hr. The length of the dioestrous phase is variable and determines the length of the cycle, while the duration of oestrus appears to be relatively constant. The figures have been compared with those published by other laboratories. Analyses of variance did not disclose any significant fluctuation in the weight of the body, adrenals or ovaries in phase with the oestrous cycle. The uterus showed a characteristic increase in weight immediately before the onset of oestrus.