- 1. One of the types of nutritive material stored in the ovum for the nutrition of the embryo; y. is particularly abundant and conspicuous in the eggs of birds. SYN: vitellus. 2. Fatty material found in the wool of sheep; when extracted and purified, it becomes lanolin. [A.S. geolca; geolu, yellow]- white y. y. consisting of much finer particles than those of yellow y.; thin layers of it lie between the zones of yellow y. and form the latebra.- yellow y. the chief constituent of the y. in a bird's egg; it consists of relatively coarse particles of stored food materials and is laid down in concentric zones with interposed thin layers of white y..
* * *yolk 'yōk n material stored in an ovum that supplies food to the developing embryo and consists chiefly of proteins, lecithin, and cholesterol
* * *n.a substance, rich in protein and fat, that is laid down within the egg cell as nourishment for the embryo. It is absent (or nearly so) from the eggs of mammals (including humans) whose embryos absorb nutrients from their mother.
* * *(yōk) [L. vitellus] 1. the stored nutrient of an oocyte or ovum. 2. crude wool fat or suint.
Medical dictionary. 2011.