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An egg yolk is the part of an egg which serves as the food source for the developing embryo inside. Prior to fertilzation the yolk together with the germinal disc is a single cell. The yolk is supplied to the egg by the mother. Mammalian embryos live off their yolk until they implant on the wall of the uterus. The egg yolk is suspended in the egg white (known more formally as albumen or ovalbumin) by one or two spiral bands of tissue called the chalazae.