Gastrulation

Gastrulation follows cleavage and the blastula stage. Through coordinated cell movements, including folding inward, spreading and migrating, the embryo reorganizes from a hollow ball or a flat disc of cells into an embryo built of three germ layers: ectoderm on the outside, endoderm on the inside and mesoderm between them. The embryo at this stage is called a gastrula, and the process also sets up its head-to-tail and back-to-belly axes.

Each germ layer has its own fate. Ectoderm forms the epidermis, hair, nails and the nervous system. Mesoderm gives rise to muscle, bone, connective tissue, the heart, blood vessels and kidneys. Endoderm forms the lining of the gut and organs such as the liver, pancreas and lungs. In many animals the inward fold creates a primitive gut, the archenteron, whose opening, the blastopore, becomes either the mouth or the anus, a difference used to separate protostomes from deuterostomes.

In humans, gastrulation begins at the start of the third week. A groove called the primitive streak forms on the upper surface of the embryonic disc, and cells migrate through it to create the new layers.