1. Mossman, A. W. (1937). Comparative morphogenesis of the fetal membrane and accessory uterine structures. Contrib. Embryol. Carneg. Inst. Wash., 26, 129
2. Barrington, E. J. W. (1964). Hormones and Evolution, p. 34 ( London: English and Universities Press )
3. Medawar, P. B. (1953). Some immunological and endocrinological problems raised by the evolution of viviparity in vertebrates. Symp. Soc. Exp. Biol., 7, 320
4. Kaye, M. D. (1971). The evolution of placentation. Aust. N.Z. J. Obstet. Gynaecol., 11, 197
5. Hagan, H. R. (1951). Embryology of the Viviparous Insects, p. 261 ( New York: Ronald Press Co. )