In the blastula, most of the blastomeres which have to become differentiated into the cells of three fundamental germinal layers- ectoderm, endoderm and mesoderm of the future embryo, occur in a single layered blastoderm and at the external surface of the blastula. To occupy their respective positions in the embryo the blastomeres of presumptive organ forming areas, move from the exterior to the interior of the blastula. Such a process of displacement of the blastomeres of the monoblastic blastula to form three germinal layers or merely the segregation of germ layers is called gastrulation and the resulted two layerd or diploblastic (e.g., Amphioxus) or three layered or triploblastic (e.g.. most vertebrates) embryonic stage is called gastrula. The gastrular embryonic state is one of the most significant stage of embryonical development of multicellular animals and basically it is characterized by following events:
1. A rearrangement of blastular cells of blastomeres by means of formative or morphogenetic movements occurs.
2. The rhythm of cellular division (cellulation or cleavage) is slowed down or almost is inhibited.
3. Growth, if any, is insignificant.
4. The type of metabolism changes and the rate of oxidation is intensified.
5. The nuclei become more active in controlling the activities of the embryonic cells. The influence of the paternal chromosomes becomes evident during gastrulation.
6. During gastrulation, chemodifferntiation is started by the synthesis of new and different kinds of protein molecules.