Compared with other mammals, the human female appears to be very extravagant with her ova. Ovulatory cycles are almost continuous from puberty to the meno-pause. In circumstances favourable to maximum fertility, the average number of births per married or cohabitating woman would be about twenty-an average that has not been even remotely reached in any society. 1 Social con-ditions not only render the majority of ovulations purpose-less, but also allow relatively infrequent non-ovulatory physiological rest-periods. Other mammals are more eco-nomical with their ova. Ovulations are limited to the breeding season, and may even occur only on demand after copulation. Moreover, the reproductive potential is exercised to the full, allowing adequate physiological non-ovulatory rest-periods. In the cow (a polycestrous animal) it is estimated that less than fifty ova are shed in a lifetime. 2 2 Data about comparative ovarian oncology have been accumulating, 3-8 and three noteworthy features have been revealed. The first is that practically all tumour types encountered in the human ovary may be seen in the mammalian ovary. The second is that ovarian tumours in other mammals are apparently much rarer than in human beings. The third is that this rarity is largely due to the infrequency of epithelial neoplasms derived from the ovarian surface epithelium-the cystadenoma and the adenocarcinoma. In women these account for the majority of all ovarian neoplasms and for the great majority of ovarian malignant neoplasms. 9