The Role of Endogenous Factors in Seasonal Maturation in Temperate Zone Female Toads, Bufo bufo

The relative importance of environmental and endogenous factors controlling the ovarian cycle was studied in juvenile female toads kept at constant temperature (210C) and a light regime of 12L: 12D or daylight. In experiments that began in October, none out of 22 toads had matured at autopsy in December, whereas 10 out of 13 toads had matured in February. In the experiment that began in April, all 16 toads had matured sexually at autopsy in June, as evidenced by the presence of complements of vitellogenic oocytes in the ovaries. In this experiment ovarian cycles were initiated within a short interval of time independently of the body size attained. The adaptation of ovarian cycles in temperate zone toads to the annual climatic cycle probably is based on the establishment of endogenous control of seasonal gonadotropic activity. In both juvenile and adult females of temperate zone toads, Bufo spp., the ovar- ian cycle, as initiated by the recruitment of a complement of oocytes to vitellogenic growth, starts in the spring or early sum- mer. If juvenile female toads attain body sizes in the late summer or autumn that would have resulted in sexual maturation early in