Gastrulation
The end of cleavage of the unicellular zygote results in the creation of multicellular blastula, which may be a solid structure with no a cavity (stereoblastula), or its cells might be arranged in the form of a one cell or several cells thick epithelium around a cavity (coeloblastula) or around or on yolk's top (Superficial blastula, Discoblastula). In either case the blastula has no similarity to the shape or organization of the body. Hence, by the consequent developmental stage the simple blastula should change itself into a more complex embryonic structure (gastrula) upon that the adult like body may be built up. Such type of a process of transformation is termed as gastrulation. It is a very important phase of ontogenetic development that marks the beginning of the development of form and organization of adult body. In the metazoans (apart from in sponges and coelenterates), the various tissues and organs of the body develop from cells that become arranged in the form of three layers, the outer ectoderm, the inner endoderm and the mesoderm among these two layers.
The three layers are termed as the germinal layers. With the exception of some parasitic flatworms a new cavity that is termed as the archenteron (future alimentary canal) is formed surrounded via endoderm. In the blastula all the cells are situated on the surface forming the blastoderm. During gastrulation there takes place displacement of the parts of blastoderm that is why the presumptive endodermal and mesodermal cells are removed from the surface of blastula and brought into the interior of embryo in which the respective organs are made in the course of further development. The cells of the presumptive ectoderm remain on the surface. So, the single layer of cells, the blastoderm, gives rise to three germinal layers viz. ectoderm, mesoderm and endoderm. Hence, gastrulation is a dynamic process involving large scale movement of blastula cells resultant in their arrangement in a way that establishes the basic body plan as per to which the embryo has to develop further. As these movements lay the foundation of the form and organization of the body they are called morphogenetic movements. They include movements of epithelial layers of cells as whole as well as independent movements of cells that break loose from epithelium and become mesenchymal.