Embryo

Embryo, name for the young of plants or animals at the earliest stage of development, after fertilization. In seed-bearing plants, the term applies to the stage before the plant emerges from its seed. In egg-laying animals, it refers to the period before hatching. In mammals, the embryonic stage lasts until the creature's basic body shape and organs are formed, at which point it is called a fetus. Animal embryos have their origin in a zygote, an ovum (egg) that has been fertilized by a sperm. By the process of cell division, the ovum forms a small solid cluster called the morula. In the next stage, a hollow cavity one-cell thick called a blastula develops. A second layer of cells develops, forming a 2-layered gastrula. In higher animals there is a third cell layer. The outermost layer, the ectoderm, develops into skin, feathers, or scales, and the nervous system. The innermost layer, the endoderm, gives rise to the lining of the alimentary canal and certain internal organs. The middle layer, the mesoderm, becomes the skeleton, muscular system, heart and circulatory system, kidneys, and reproductive organs. Exactly how a set of virtually identical cells develops into a great variety of specialized tissues remains one of biology's mysteries.

See also: Birth.