When fertilized and unfertilized eggs of Arbacia punctulata are suspended in heavy water, deuterium is incorporated into stable positions in the egg proteins. The rate of incorporation of the isotope is considerably greater in fertilized than in unfertilized eggs, and is accelerated at the time of formation of the blastula. The result of calculation of the maximum deuterium concentration which would be reached on complete turnover indicates that at least one out of every ten stably bound hydrogen atoms of the egg proteins is a deuterium atom. This has been interpreted as evidence that at the time of formation of the sea urchin blastula and in the period of development which follows, synthesis and breakdown are simultaneous processes leading to the redistribution of amino acids among the egg proteins.

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