Gastrula

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1 - blastula, 2 - gastrula; orange - ectoderm, red - endoderm.

The gastrula phase of embryonic development is seen in all animals except the sponges. It follows the blastula phase.

The purpose of gastrulation is to position the three embryonic germ layers, the endoderm, ectoderm and mesoderm. These layers later develop into certain bodily systems.

The ectoderm develops into the brain, skin, nails, and other epidermis associated structures.

The endoderm developes into the inner linings of the digestive tract, as well as the linings for the respiratory tracts. It also forms many glands, such as the liver and pancreas.

The mesoderm forms the somites, the notochord, and the mesenchyme.

During gastrulation, embryonic cells migrate through an opening within the embryo known as a blastocoel. As the gastrula forms, the remnants of the blastocoel shrink to eventually disappear completely.

The opening into the gastrula is known as the blastopore. The inner cavity created by the infolding is known as the archenteron.

There are five main types of cell movements in gastrulation:

ingression - the movement of single cells inwards
involution - the inturning of a lower cell layer caused by movement of the upper layer
invagination - an infolding, or poking, of cells
delamination - when one sheet of cells split into two
epiboly - when the embryo is encompassed by the ectoderm.
In addition to these movements, convergent extension can also take place. Although it is not real movement it does allow the cells to stretch (shorter, longer, or taller)

Once gastrulation is complete, all germ layers are in the correct location and further growth and organogenesis begins.