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1. Chick embryos with primary circulation, up to about 3 days of development, show no hemoglobin-mediated transport of oxygen. 2. In embryos with secondary circulation, between 3 and 6 days of incubation, the vascular area acts as the respiratory organ. Its efficiency in the oxygen uptake is less than that of the chorioallantois of later embryos. On the contrary, oxygen release to the tissues is highly efficient. 3. A full efficient hematic uptake of oxygen is reached at about the 6th incubation day, when chorioallantois acts as the embryonic respiratory organ. 4. The different respiratory mechanisms of developing chick embryo are closely related to the functional properties of the various hemoglobins which are produced during the embryonic life.

1. Early chick embryos (up to 7 days) survive longer in 1% CO than older ones. As shown by the partition constants of the oxygen-carbon monoxide equilibrium, such a behavior cannot be related to the different hemoglobin patrimonies. 2. In vivo, early embryo hemoglobin is completely turned into its CO derivative within about 15 min according to a step-wise progression which reflects the peculiar embryonic pathway of the blood circulation. 3. Diffusing oxygen is sufficient to keep early embryos alive for a long time. 4. Before the chorioallantois is formed, the vascular area acts as the embryonic respiratory organ.