卵(egg)

在生物学上，指雌性的性细胞或配子（gamete）。在动物学上，卵一词可用於指包括雌性细胞、多种保护膜（protective membrane）及营养物质的整个特化结构，而来自拉丁文的卵子（ovum）一词常用以指单个雌性性细胞。卵或卵子与雄性配子（精子〔sperm〕）一样，仅具有单倍染色体（单倍体〔haploid〕，参阅ploidy）；雌性和雄性配子受精（fertilization）後即结合形成含双倍染色体（双倍体〔diploid〕）的合子（受精卵〔zygote〕）。在人类，卵子在卵巢的卵泡（ovary's follicle）里生长成熟，然後随卵泡破裂释出（排卵〔ovulation〕），进入输卵管准备受精；如果卵子排出後约24小时内未能受精就将退化。多种动物的卵含有大量的营养物质（卵黄〔yolk〕），其多少取决於幼体能自行觅食的时间；在哺乳动物（mammal）则取决於幼体从母体循环获得营养的开始时间。大多数动物的卵都有一或多层膜包绕，昆虫（insect）的卵有一层又厚又硬的卵壳，两栖类（amphibian）的卵有一层胶冻样物质。鸟和爬虫类的卵细胞外有硬壳，这些结构统称为蛋。

English version：

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 (uterine) 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.

参考文章

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