The study of the effects of exogenous glucagon on glycogen concentrations in various organs of the developing chick embryo has been extended to the yolk sac membrane. Three series of eggs were prepared. Normal yolk sac membrane glycogen concentrations were determined at daily intervals from day 3.00 to 20.00 of incubation. Glucagon (300 μg) was administered to embryos on days 3.00, 4.00, 6.00, 8.00, 10.00, 12.00, 14.00, and 16.00 of incubation. Controls received appropriate amounts of glucagon-vehicle. Normal yolk sac membrane glycogen levels remain relatively constant during the first 2 weeks of incubation. However, during the incubation interval of 14–16 days, the glycogen levels have increased, and by the 18th day of incubation the glycogen concentration has more than doubled that of day 14. In contrast, the period of 18–20 days of incubation is characterized by a precipitous drop in this yolk sac membrane polysaccharide. In the experimental series, statistically significant decreases of yolk sac membrane glycogen levels occurred 0.25 day after glucagon administration in all intervals studied with the exception of days 3.00 and 16.00. On day 3.00 the glycogen levels were unaffected by hormonal addition, while on day 16.00 the depletion of yolk sac membrane glycogen occurred at 0.125 day with a return to normal by 0.25 day. The other recovery times varied from 0.50 to 1.00 day posthormone addition with the exception of day 8.00, where recovery had not occurred in the first 24 hours. The observation of Dieterlen-Liévre [ Compt. Rend. Acad. Sci., 256, 1597 (1963)] that 3-day dorsal pancreatic anlage, when grown in juxtaposition to 6-day chick embryo liver inhibits glycogen formation, coupled with the data of the present investigation, suggests that the chick embryo may well exert hormonal control over glycogenolysis in the yolk sac membrane.