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.
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