Embryo

Embryo
A human embryo, seven weeks old or from the ninth week of pregnancy.
Identifiers
TE	E1.0.2.6.4.0.8
FMA	69068
Anatomical terminology []

An embryo is an early stage of development of a multicellular diploid eukaryotic organism. In general, in organisms that reproduce sexually, an embryo develops from a zygote, the single cell resulting from the fertilization of the female egg cell by the male sperm cell. The zygote possesses half the DNA of each of its 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.

In humans, a pregnancy is generally considered to be in the embryonic stage of development between the fifth and the eleventh weeks after fertilization, and is expressed as a fetus from the twelfth week.

First attested in English in the mid-14c., the word embryon derives from Medieval Latin embryo, itself from Greek ἔμβρυον (embruon), lit. "young one", which is the neuter of ἔμβρυος (embruos), lit. "growing in", from ἐν (en), "in" and βρύω (bruō), "swell, be full"; the proper Latinized form of the Greek term would be embryum.

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 , surrounded by a sphere or sheet of cells, also called blastomeres. In a placental mammal, an ovum is fertilized in a fallopian tube through which it travels into the uterus. 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 eleventh 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 later stages.