Observations upon developing living mammalian foetuses have been restricted” mainly to their movements.1, 2 Since earlier stages of development offer definite information concerning the mechanics of mammalian morphogenesis, the problem of embryonic survival under varied conditions is of utmost importance. With this in mind the following experiments have been performed.The rat embryos (8 and 9 days of gestation age) were carefully dissected free from the uterus and the decidua capsularis removed. The embryonic membranes which are just forming at the latter stage were left intact. The embryos were placed in Ringer's, Locke's, and physiological saline solutions and their morphological reactions noted. They were removed from the solutions at various times and fixed for histological study.While the survival time varied considerably, no signs of histolysis were apparent after 6 hours in any of the above solutions at 37.5°C. When kept at 24°C. in the same solutions there was no sign of disintegration 12 hours af...