Why Is the Egg Yolk Known as the Food Store? When an animal lays eggs, the yolk is the portion of the egg that contains nutrients and serves primarily to feed the developing embryo. Yolk is the food store in animal or bird eggs, which consists of protein and fats. In chicken eggs, yolk is the yellow part we see when we break an egg.
Certain egg types don’t have yolks because they develop in the parent’s body, which provides the food, usually through the placenta, or because they are placed in environments where there is a plentiful supply of food, such as the body of a parasitoid host.
In many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. The yolk is not always enclosed within the egg shell, however. In some invertebrates and fish such as trout, the yolk material is not in a special organ, but inside the egg cell, the tiny hatched fish swim at first with yolk sacs suspended beneath their throats.
Yolk is frequently high in proteins, fats, minerals, and vitamins since they are stored food. In addition to serving as food in and of themselves, proteins also control how other nutrients are stored and supplied. For instance, in certain species, the amount of yolk in an egg cell influences the post-fertilisation developmental stages.
Unlike protoplasm, which is live cell material, the yolk is primarily passive material, or deutoplasm. Oogenesis provides the food material and related control structures. A portion of the material is stored essentially in the form that the mother body provided it, partially processed by specific non-germ tissues within the egg, and partially biosynthesized into its ultimate form within the oocyte itself.
Other than animals, certain organisms, such as algae, particularly those that are oogamous, can store nutrients in the gametes of their females. The endosperm of blooming plants and the remnants of the female gametophyte in gymnosperms are both sources of nourishment.
Content for this question contributed by Mischelle Thomas, resident of Umatilla, Umatilla County, Oregon, USA
