Gravitationally induced displacements of the contents of the frog egg can predictably determine the orientation of the subsequent dorsal-ventral axis of the embryo, regardless of the original position of sperm entry or of the grey crescent. In certain conditions, these displacements in the egg can also lead to the formation of a second axis, that is, to twinning. The previously reported ability of grafts of grey crescent cortex to induce secondary axes in recipient eggs is interpreted here as an unrecognized twinning effect of gravity. Our results lead to question the classic interpretation of the grey crescent as a dorsal determinant in amphibian development.

To test whether gravity is required for normal amphibian development, Xenopus laevis females were induced to ovulate aboard the orbiting Space Shuttle. Eggs were fertilized in vitro, and although early embryonic stages showed some abnormalities, the embryos were able to regulate and produce nearly normal larvae. These results demonstrate that a vertebrate can ovulate in the virtual absence of gravity and that the eggs can develop to a free-living stage.Is gravity required for normal embryonic development? Upon fertilization, most amphibian eggs rotate inside the fertilization membrane so that the animal-vegetal axis is aligned with gravity. This "rotation of fertilization" (1) is not a requirement for normal development, since eggs prevented from rotating can develop normally (2). Nevertheless, the direction of the rotation of fertilization normally has a role in determining the polarity of the embryonic axis (1), and eggs inclined with respect to gravity form the dorsal structures on the side of the egg uppermost in the gravitational field (3, 4). Thus, for over a century scientists have questioned whether gravity was required for amphibian embryogenesis (5, 6). Recent spaceflight experiments have successfully fertilized eggs of Xenopus laevis and reared the embryos to the gastrula stage. The morphology of the gastrulae was somewhat abnormal (7). We report here that ovulation and subsequent development to a free-living stage can occur at microgravity (<10-3 X g). We also examined larval swimming behavior to determine whether abnormal behaviors of Xenopus tadpoles previously observed during and following space flight (8) would occur with larvae that began their lives in the virtual absence of gravity.MATERIALS AND METHODS Preflight Procedures. X laevis frogs were obtained from J.

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