Nutrients required for the growth and development of an avian embryo must be present when the egg is laid. Many, if not most, of the nutrients in eggs are transferred from the blood plasma of the hen into the yolk of the oocyte as specific nutrient-protein complexes. Egg yolk contains vitamin-binding proteins for thiamin, riboflavin, biotin, cobalamin, retinol, and cholecalciferol. The biochemical details of how these plasma vitamin-protein complexes are recognized by and deposited in the oocyte and subsequently dissociated for use by the embryo are not known. Niacin and ascorbic acid are synthesized by the embryo from other compounds deposited in the egg. Pantothenic acid, which is abundant in the egg, is not bound tightly to a specific protein. Binding proteins for thiamin, riboflavin, biotin, and cobalamin are also present in egg white. Because they are usually not saturated with respect to their ligand, these binding proteins are able to scavenge nutrients and thereby are thought to protect the embryo from infection by microbes that require the ligands. In the albumen of a few species, nutritionally significant amounts of bound riboflavin or biotin are present, suggesting both nutritional and antimicrobial functions for their binding proteins. It is postulated that differences in the amounts of various nutrient-binding proteins correspond to differences in the nutrient contents among the eggs of various species and reflect differences in the nutrient needs of the contained embryos. Mutations that inactivate nutrient-binding proteins arrest development before hatching and are dependent solely on the maternal genotype.