Gastrulation varies in different phyla. It is followed by organogenesis, when individual organs develop within the newly formed germ layers.
The gastrula is the double-walled stage of the embryo. During gastrulation, some of the cells of the blastula migrate inwards to make an inner layer, the endoderm. In some animals another cell layer forms, the mesoderm, between the endoderm and the ectoderm.
Among animals, sponges show the simplest organization, having a single germ layer. Cnidaria and ctenophores show an increase in complexity, having two germ layers, the endoderm and ectoderm. All higher animals (from flatworms to humans) possess a mesoderm as well as ectoderm and endoderm.
Development of germ layersEdit
The purpose of gastrulation is to position the 3 embryonic germ layers, the endoderm, ectoderm and mesoderm. These layers later develop into certain bodily systems.
- The ectoderm develops into the 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. This key innovation evolved hundreds of millions of years ago and led to the evolution of nearly all large, complex animals. The formation of a mesoderm led to the development of a coelom. Organs formed inside a coelom can freely move, grow, and develop independently of the body wall while fluid cushions and protects them from shocks.