Embryo

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An embryo (irregularly from Greek: ἔμβρυον, plural ἔμβρυα, lit. "that which grows," from en- "in" + bryein "to swell, be full"; the proper Latinate form would be embryum) is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination. In humans, it is called an embryo until about eight weeks after fertilization (i.e. ten weeks LMP), and from then it is instead called a fetus.

The development of the embryo is called embryogenesis. In organisms that reproduce sexually, once a sperm fertilizes an egg cell, the result is a cell called the zygote that has half of the DNA of each of two parents. In plants, animals, and some protists, the zygote will begin to divide by mitosis to produce a multicellular organism. The result of this process is an embryo.

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Animal embryos

In animals, the development of the zygote into an embryo proceeds through specific recognizable stages of blastula, gastrula, and organogenesis. The blastula stage typically features a fluid-filled cavity, the blastocoel, surrounded by a sphere or sheet of cells, also called blastomeres. The embryo of a placental mammal is defined as the organism between the first division of the zygote (a fertilized ovum) until it becomes a fetus. In humans, the embryo is defined as the product of conception from implantation in the uterus through the eighth week of development. An embryo is called a fetus at a more advanced stage of development and up until birth or hatching. In humans, this is from the eighth week of gestation. However, animals which develop in eggs outside the mother's body are usually referred to as embryos throughout development, e.g. one would refer to a chick embryo, not a "chick fetus" even at late stages.

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