Changes in blood PCO2 and acid-base status in chick embryo between day 4 and 6 of incubation.

PCO2 was measured in extraembryonic blood vessels of early chick embryos (first third of incubation period) using PCO2 microelectrodes with liquid ion exchanger. Between day 4 and day 6 of incubation mean PCO2 in the vitelline veins increased from 4.2 to 7.0 torr and in the vitelline arteries from 6.9 to 10.6 torr. From the CO2 titration curves obtained in ovo by measurements in hypercapnic environment buffer values of the blood were evaluated (28.8 mmol/l at day 4 and 21.4 mmol/l at day 6). Calculated standard bicarbonate concentration was 29.4 mmol/l at day 4 and decreased to 24.4 mmol/l by day 6 indicating metabolic acidification. These results are compatible to the data previously determined for older embryos.