Superfetation in viviparous Cyprinodont fishes

" Although viviparity is the rule in the marsupial and placental mammals and occurs also in some reptiles and selachians, superfetation does not take place. The ovocytes reach maturity seasonally or periodically within the ovarian follicles, after which ovulation occurs. The ova enter the oviducts where fertilization takes place and the developing embryos are then retained in the oviduct or uterus for the period of gestation. In the mammals, at least, the implantation of the embryos in the uterus brings about changes in the endocrine control of ovulation and the cyclical changes in the uterus with the result that ovulation does not occur during gestation and the cyclical changes in the uterus which proceed in the absence of fertilization of the ovum are held in abeyance. In the viviparous reptiles and selachians large yolk-laden eggs are developed seasonally. After ovulation the ova are drawn into the oviducts where they are fertilized and the embryos are then retained in the oviducts during the period of gestation. In the meantime new ovocytes are developing in the ovary but they do not become mature by the end of the gestation period. Some time elapses, therefore, between the birth of one group of embryos and the ovulation and fertilization of the ova which are to form the next brood "