Daily pairing of normal adult female gerbils with indicator males revealed an oestrous cycle of 4 to 6 days. Under the conditions of the experiment, oestrous behaviour usually began between 12.00 and 17.00 hours and lasted less than 24 hr (probably 12 to 15 hr). The appearance, in vaginal smears, of small cornified elements termed `transitional cells' was partially useful in predicting the approach of receptivity. However, the vaginal smear was reliable only as a general indicator of approaching heat, and gave no temporal information.
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