yolk /yōk, yelk/ [AS, geolca] , a material rich in fats and proteins that is contained in an ovum and that supplies nourishment to the developing embryo. The amount and distribution of the yolk within the ovum depend on the species of animal and the type of reproduction and development of offspring. In humans and most other mammals, yolk is absent or greatly diffused through the ovum. Mammalian embryos absorb nutrients directly from the mother through the placenta. Also called vitellus. Compare deutoplasm.
