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In Heterandria formosa, a viviparous cyprinodont fish of the family Poecitiidae, nearly all of the nourishment for embryonic development is obtained from the mother after fertilization through a pseudoplacentat association (Scrimshaw, 1944). Similar although less complicated associations between mother and embryo exist in other species of this family (Turner, 1937, 1940) . These species are for the most part considered to be ovoviviparous. The distinction is based on the difference between live bearing forms which retain an egg with a full supply of nourishment for development and those whose embryos receive nourishment froni the mother. The latter are considered truly viviparous. Turner (1937) referred to all species of Poecitiidae as bvoviviparous. He obviously used this term in a general sense for he suggested in discussing the data of Bailey (1933) that Xiphophorus helleri could receive nutriment from the parent. Turner also pointed out (1937) that the small Heterandria egg cannot contain enough nourishment to account for the size of the larvae and suggested that the follicle cells surrounding the embryo furnish food materials. The weight values reported by Bailey for various embryonic stages in Xipho phorus show no decrease in the weight of the total yolk-embryo system. Since energy is used for maintenance metabolism, the total weight of this system de creases in forms depending entirely on yolk. Gray (1928) reported a decrease of 37 per cent for the oviparous trout, Salnto fario, and Hsiao (1941) found a de crease of 34 per cent in the truly ovoviviparous perch, Seba.stes marinus. Accord ingly, although he does not suggest this, Bailey's data show that some nourishment must be obtained from the mother by the developing embryo of Xiphophorus. It is true that most poeciliid fishes are more dependent on the yolk laid down before fertilization than upon maternally supplied nourishment after that time. However, the evidence presented below shows that the members of this family do utilize nourishment outside of that contained in the yolk and hence are not ovo viviparous in the strict sense of the term. Embryos of such species as Heterandria formosa and Aulo phallus elongatus are truly viviparous and as dependent on the mother for nourishment as are those of a placental mammal. Gray (1926, 1928) reported the relationships between nourishment and growth rate in the oviparous trout, Salmo fario. The problems of oviparity and ovo viviparity are similar in that in each the embryo has its own supply of nourishment and receives food and water from its environment. When these conditions are compared with true viviparity, striking differences are noted (Scrimshaw, 1944). The present study was undertaken to find and describe intermediate stages between ???? Now at the School of Medicine and Dentistry, University of Rochester, Rochester, New York. The author gratefully acknowledgesthe advice and assistance of Dr. Leigh Hoadley of the Harvard Biological Laboratories.