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(Micro-)gravity actions as identified From developmental biology experiments in Space and their interpretation.   H.-J. Marthy, CNRS, Observatoire Ocйanologique, 66650 Banyuls sur mer, France. 

Looking backwards in time, the hypothesis appears still well justified that crucial events in animal development, such as fertilization, body pattern lay out, germ layer formation or particular organogenetic processes could hardly occur in a correct manner in the absence of gravity. It was (and still is) astonishing, therefore, to learn from various experiments, performed on various aquatic and terrestrial organisms in development,  that the morphogenetic role of gravity is minor or  even neglectable. Thus, in all cases studied so far (amphibians, fishes, insects, echinoderms), viable juveniles result from eggs or embryos maintained in real micro-gravity conditions for a portion or  for the whole period of development. Inversely, in all these cases, some deviations from the “normal” way of development are reported, giving evidence that eggs and embryos “sense” apparently a particular gravity situation as well known for “single-(animal and human )-cell systems in vitro”. In contrast yet, the proliferating and differentiating “multi-cellular organisms” seem capable to overcome the perturbations and achieve, supposedly due to coordinating intercellular signals (regulation signals), the state of viable and finally reproducible individuals. From the reported morphological and physiological deviations, no far reaching generalizations as to the morphological role of gravity in animal development can be made. However, a few, apparently (micro-)gravity related phenomena can be identified in the different models, which allow similar statements. Among others, these are in particular: - early developmental stages are more sensitive against gravity alteration  than more advanced ones - cell adhesiveness appears gravi-sensitive and - “multi-cellular systems” regulate morphogenetic deviations throughout development, which leads to viable juveniles. The phenomena, supporting such conclusions, are overviewed in  the different models.

(Support to the author is provided by CNES and CNRS of France.)