The effect of steroid pre-treatment upon progesterone-induced ovulation. Frogs were pre-treated with ethanol (Et), estradiol (E) or progesterone (P) at the indicated concentration for a period of one day before induction by progesterone. The left column represents induction results in response to PMSG-hCG injection. After 24 to 30 hr incubation, the number of ovulated eggs was counted as described in the Materials and Methods. Annotations underneath each column indicate substance and concentration for both pre-treatment and the induction of ovulation. For example, Et0-P20 represents pre-treatment with 0% ethanol for one day followed by induction by a mixture of 20 μM progesterone the following day. Each value represents the mean of data from more than three different females. The number of ovulated frogs per number treated is indicated over each column. Vertical lines indicate standard deviation.