A chalaza (plural chalazae) is a structure inside an egg that helps to keep the yolk in place. It attaches to either end of the yolk and anchors it to the inside of the eggshell, essentially suspending the yolk. These structures prevent the yolk from being damaged, promoting the healthy development of the embryonic bird. They are also present in some plants, performing a similar function in plant ovules.
Many people who cook with eggs have found a chalaza or two in their work, and they look like a little stringy white rope inside the egg. Sometimes, both are visible, especially in fresh eggs, and in other cases, only one can be found. The structure is perfectly safe to eat, although some people remove it because they are concerned about its impact on the texture of a dish. A fine custard, for example, might be disrupted by its stringiness.
Originally, the chalaza starts out like a thin string. Over time, the structure usually becomes twisted, as the yolk moves around inside the egg. The chalazae develop a spiral pattern, exactly like a string that has been repeatedly twisted, and the twists stay in place because of the weight of the yolk prevents them from unwinding.
In addition to this structure, and the yolk and the white with their respective membranes, eggs also include a thin membrane between the white and the shell, and an air cell between the membrane and the egg, which develops as the contents of the egg shrink. Over time, the air cell becomes larger, and a big one can be a sign that an egg is old.
The fresher an egg is, the more noticeable the chalazae are. Some cooks regard a prominent set as a sign of very high quality, although they only really reflect freshness. For quality, the color of the yolk has to be considered, and the darker the yolk, the more nutritious the egg. Pale yolks indicate poor nutrition, and these eggs tend to not perform as well in cooking and baking. They may have trouble, for example, acting as binders in a dish, and they also provide fewer nutrients to the consumer.
Chalazae are sometimes assumed to be the start of an embryo, but they are not. Fertile eggs can actually be identified by a small dark spot and signs of red veining on the yolk.