Abstract Stages 20 and 25 chick apical ectodermal ridge have been cultured in nutrient medium containing fetal bovine serum and the tissues have been examined for dying cells at 0, 6, 12, 18, and 24 hr. By 12 hr, an average of 43% of the cells were dying. By 24 hr, stage 20 ridge had lost its integrity and stage 25 ridge contained an average of 50% dying cells. These results are in agreement with the observations of R. L. Searls and E. Zwilling (1964, Dev. Biol. 9, 38–55) on isolated stage 20 ridge. In subsequent experiments, ridge ectoderm was cultured in serum-containing medium to which insulin (5 μg/ml), transferrin (5 μg/ml), and selenium (5 ng/ml) or insulin (5 μg/ml) had been added. Under these conditions the ectoderms remained viable even after 24 hr in vitro.