Oxygen availability and growth of the chick embryo

Two experiments were performed using White Leghorn eggs incubated in 50% humidity, at 38 degrees centigrade, and an average barometric pressure of 747 Torr. In one experiment, eggs in the experimental group were each half covered with a neoprene membrane, reducing the diffusing capacity by approximately 20%, and incubated in 21% O2. In the second experiment, the experimental eggs were incubated (uncovered) in 60% O2. Control eggs were incubated (uncovered in 21% O2 simultaneously with the experimental eggs in each study. A relationship between egg surface area (calculated from weight of the freshly laid egg) and embryo weight at 18 days was confirmed for all three groups. When comparisons were made among the groups, there was significant retardation of embryonic growth in the half-covered eggs; in contrast, embryos from eggs incubated in 60% O2 were significantly heavier at 18 days that control eggs incubated in air. The fact that embryo growth can be accelerated by incubation of the egg in 60% O2 suggests that the oxygen tension in blood leaving the chorioallantoic capillaries normally limits the rate of embryonic growth for the first 18 days of incubation in this species.