An embryo (from ancient Greek ἔμβρυον / émbruon) is a developing since the first division of the egg or zygote up to the stage where major organs are formed.

The embryology is the study of the growth of a body during gestation, ie at a time during embryonic and fetal period.

Teratology is the study of birth defects and congenital anomalies.

In humans
In humans, the embryonic stage lasts eight weeks or ten weeks of amenorrhea. This period corresponds to the formation of the fetus. Beyond and to the end of pregnancy, it is called a fetus.

Embryo development in metazoans
The developing embryo of most multicellular animals, or metazoans, comprises successively:

* Cleavage or segmentation that corresponds to the first mitoses of the embryo.

* Gastrulation, which sets up the two (ectoderm, endoderm) or three (+ mesoderm) sheets which will form embryonic tissues and organs characteristic of the species. The organization of the embryo is often disrupted during this process which involves many cell migration.

* Neurulation and organogenesis during which the nervous system and organs are formed from sheets.

The embryos of metazoans can breathe either by direct diffusion through the integument (eg amphibian embryos) or with an annex that embryonic plays the role of "lung" or "industry" transitional (d embryos 'amniotes, for most mammals the placenta is providing this role).

The embryos of metazoans can feed from the reserves in their cells initially deposited into the oocyte by their mothers (embryos of arthropods, amphibians), or feed with embryonic annexes (d embryos 'amniotes, for most mammals the placenta is providing this role).

In plants
In botany, the embryo is the set of cells resulting from division of the egg until the seedling stage, the body being kept alive latent variable for a time, within the seed and consists of four parts: the gemmule, shoots, (s) cotyledon (s) and the radicle.

The polyembryonie
The polyembryonie is defined as the formation of at least two viable embryos (or more) from a single fertilized egg, forming what are known as identical twins in humans.

The plant polyembryonie
In plants, some seeds contain two zygotic plants, ie an embryo resulting from gamete fertilization (very rare), two genetically different plants: an embryo is an embryo nucellaire (clone of the mother "born without fertilization and holder of 100% of maternal genes ... in theory, this will just push the fruit initial) and the other is from fertilization with another flower (pollen ovule +, 50% with 50% all hazards arising fertilization without counting the possible hybridizations).

The polyembryonie allows to regenerate a full variety of viruses (which do not normally in the embryo nucellaire) or to obtain clones of the rootstock.

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