Induction probably requires the presence of certain chemicals that turn genes on. Somites arise from the mesoderm. These become the muscles of the body and the vertebrae. A body cavity called the coelom forms and is lined by mesoderm. The coelom becomes the thoracic and abdominal cavities.
Extraembryonic membranes extend out beyond the embryo.
The amnion envelops the fetus in protective fluid. The yolk sac is the first site of red blood cell formation, and part of this membrane eventually becomes a portion of the umbilical cord. The allantois contributes to the circulatory system, and its vessels become the umbilical blood vessels. The chorion, the outermost membrane, contributes to the placenta. The chorion contributes the placenta on the fetal side, while uterine tissues supply the placenta on the maternal side.