Sodium thìocyanate action on the first stages of chick embryo: The early stages of chick embryos tolerate 0.005 M NaSCN. Acting during primitive streak formation (action in egg), this salt induces a certain number (about 17%) of embryos with enlarged notochord (fig. 1; Plate II, figs. 6, 7, 8, 9, 10, 11). Also the number of notochord cells increases (fig. 2). The volume of these cells is increased too. In some of these embryos with large notochord, the somites appear to be reduced in volume (Plate IV, figs. 20, 21), following the Amphibian process in which some cells of the presumptive somite rudiment seem to become notochord. Besides this action, the NaSCN disaggregates the somites (Plate III). This disaggregation has been observed in embryos treated since the definitive primitive streak stage (action in vitro). In the neural tube, in embryos with normalnotochord, it is possible to observe cell masses that remain enclosed in the neural tube cavity (Plate II, figs. 12, 13), and correspond to the ganglion rudiments observed in Amphibia. Hydrops are not infrequently observed in the embryo body (Plate IV, fig. 23). A dilatation of the first aortic arch often presses on the brain (Plate I, fig. 2; Plate III, fig. 14; Plate IV, fig. 24).