The embryo ofMegaderma lyra lyra enters the uterus as a morula. The blastocyst establishes contact with the uterine wall circumferentially at the level of implantation. The orientation of the embryonic mass at early stages is variable, but the flat embryonic disc at later stages is invariably mesometrially oriented. The preimplantation response of the uterus is markedly evident in the enormous expansion of the uterine glands and the hypertrophy of their cells. After the attachment of the blastocyst there is a rapid degeneration of the uterine glands and the cells of the endometrial stroma resulting in the formation of a thick zone of symplasma into which roots of the trophoblast enter. The maternal blood capillaries increase in number and their endothelial cells undergo hypertrophy. The amniotic cavity is formed by cavitation, but with the expansion of the embryonic disc the original roof of the amnion undergoes rupture and loss. The definitive amnion is formed by the development of folds from the margin of the embryonic disc.