noun, plural: eggs
(1) An organic vessel where an embryo develops, and one in which the female of an animal species lay as a means of reproduction
(2) An ovum
An egg is an animal reproductive body that is spherical or ellipsoidal in shape. Inside the egg is the fertilized ovum that grows and develops as an embryo. It is produced and released outside of the animal body by fish, birds, snakes, insects, turtles, and other animals. A bird’s egg, for instance, is typically comprised of the following parts: (1) shell, (2) thin, proteinaceous membranes, (3) air cell, (4) albumen, (5) chalazae, (6) vitelline membrane, (7) yolk, and (8) zygote. Eggs may be classified according to the relative amount of yolk:
- microlecithal egg
- mesolecithal egg
- macrolecithal egg
Inside the egg, the zygote forms a blastodisk, which develops into an embryo, and finally into a fetus. The developing young uses the yolk for its sustenance. This kind of reproduction (i.e. laying eggs) is called oviparity in contrast to viviparity wherein the young develops inside the mother’s womb, derives nourishment from the mother, and is born alive.
Eggs laid by certain animals, such as bony fish, are not yet fertilized. These eggs are fertilized externally and this kind of egg-laying reproduction is termed ovuliparity.