1. Embryos of the domestic fowl have been incubated in normal conditions and in an atmosphere of 9 % carbon dioxide from day 9 onwards.

  2. There is a rapid increase in blood bicarbonate and base excess when the embryos are exposed to carbon dioxide. The increase amounts to about 14 m-equiv./l and is relatively constant from day 12 onwards.

  3. There is no detectable increase in titratable acidity or ammonium ion content of allantoic fluid which would indicate that the extra plasma bicarbonate ions were formed by renal activity.

  4. It is suggested that the extra bicarbonate of the blood is derived by resorption of eggshell minerals.

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