Embryo

An embryo is a young organism during the period when its body plan and main organs are being laid down. In animals the word covers development from the first divisions of the zygote until the young hatches or is born, or in mammals until it becomes a fetus. In human medicine, the embryonic period runs from implantation in the second week to the end of the eighth week after fertilization; from the ninth week the developing human is called a fetus. In plants, the embryo is the young plant inside a seed.

During the human embryonic period, the inner cell mass of the blastocyst flattens into a disc that undergoes gastrulation in week 3, forming the three germ layers: ectoderm, mesoderm and endoderm. Neurulation forms the neural tube, the heart starts beating early in week 4, limb buds appear, and by week 8 the rudiments of all major organs are present, though the embryo is only about 3 cm long. Meanwhile the placenta and the membranes around the embryo, including the amnion, develop to support it.

Because organs are forming, this is when the embryo is most sensitive to teratogens, agents that cause birth defects, such as some drugs, alcohol, certain infections and radiation.