Gastrula

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

Development
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, the epithelium of the nose, mouth and anal canal; the lens of the eye, the retina and the nervous system.


 * The endoderm develops into the inner linings of the digestive tract, as well as the linings of the respiratory passages. It also forms many glands, such as the liver and pancreas.


 * The mesoderm forms the somites, the notochord, and the mesenchyme, which give rise to the muscles, circulatory and excretory systems of the body.

During gastrulation, embryonic cells migrate through an opening within the embryo known as the 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.

Movements
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, organogenesis begins.