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egg
In biology, the female sex cell, or gamete. In zoology, the Latin term ovum is often used to refer to the single cell, whereas the word egg may be applied to the entire specialized structure or capsule that consists of the ovum, its various protective membranes, and any accompanying nutritive materials. The egg or ovum, like the male gamete (sperm), bears only a single (haploid; see ploidy) set of chromosomes. When female and male gametes unite during fertilization, the double (diploid) set of chromosomes is restored in the resulting zygote. In humans, the ovum matures inside one of the ovary's follicles (hollow group of cells) and is released when the follicle ruptures (ovulation). The ovum passes into the fallopian tube and will degenerate if not fertilized within about 24 hours. In animals, the amount of nutritive material (yolk) deposited in an egg is dependent on the length of time before the developing animal can feed itself or, in the case of mammals, begins to receive nourishment from the maternal circulation. Most animal eggs are enclosed by one or more membranes. Insect eggs are covered by a thick, hard outer membrane, and amphibian eggs are surrounded by a jellylike layer. The term egg also refers to the content of the hard-shelled reproductive body produced by a bird or reptile.
egg
egg [eg]
Egg (fowl)
A single, large, living, female sex cell enclosed in a porous, calcareous shell through which gases may pass. Although they vary in size, shape, and color, the eggs of chickens, ducks, geese, and turkeys are essentially the same in structure and content (see illustration). Inward from the shell are the outer and inner shell membranes which are also permeable to gases. The membranes are constructed to prevent rapid evaporation of moisture from the egg but to allow free entry of oxygen, which is necessary for life. Air begins to penetrate the shell soon after the egg is laid, and it tends to accumulate in a space between the two membranes at the large end of the egg. See Cell (biology)
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The inner shell membrane surrounds a mass of fluid albumin which, in turn, encloses a body of dense albumin; these two types of protoplasm constitute the so-called egg white. The central part of the egg is occupied by the yolk, which contains the vital egg nucleus and its associated parts. The yolk consists of alternating layers of yellow and white yolk. The yolk, enclosed by the vitelline membrane, is held in place by the chalaza which is anchored at each end of the egg and prevents undue mechanical disturbance. See Cell nucleus, Yolk sac
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