AbstractVegetalization was produced in sea urchin embryos by treatment with D‐ and L‐p‐tyrosine, o‐tyrosine, and m‐tyrosine, for 2 to 6 hours during cleavage. Glycine was without effect on development; phenylalanine produced general effects represented by reduction of both animal and vegetal structures. The vegetalizing action of tyrosine was synergistic with that of lithium or dinitrophenol. However, unlike the latter compounds, tyrosine had no effect on respiratory rate of the embryos.The uptake and the incorporation of algal hydrolysate‐C‐14 and leucine‐C‐14, added to the sea water medium, were lower in sea urchin embryos in the presence of tyrosine or phenylalanine than in untreated control embryos. Inhibition of incorporation caused by phenylalanine was about twice that by tyrosine, and it was postulated that the different effects of these substances on development are related to the degree of inhibition. Neither glycine nor lithium inhibited amino acid uptake.When embryos were treated with tyrosine in the presence of a full complement of those free amino acids normally found in the egg, the vegetalizing effect of tyrosine was annulled. Since the sea urchin depends upon endogenous amino acids for protein synthesis during cleavage, it is suggested that in some way tyrosine limits the endogenous supply of amino acids available for synthesis of the proteins associated with animal structures by inhibiting their release fro