During the early phase of the development of most animal embryo, gastrulation takes place. The primary germ layers which are ectoderm, endoderm, and mesoderm are formed and organized in their proper location during gastrulation. The process begins when a hollow, single-layered cluster of cells called Blastula undergoes reorganization to form a two or more germinal layer of cells called Gastrula. The germ layers are; the endoderm which is the internal layer, it develops into the gut-lining and other internal organs; the mesoderm which is the middle layer, it develops into the muscular, circulatory and skeletal system; and the ectoderm which is the outermost layer, it develops into the skin, brain, nervous system and other tissues.
The hollow blastula which is a simple epithelial layer (the blastoderm) invaginates into the blastocoel to form a doubled-wall cup structure. The cavity that is formed is called the archenteron while its open-end is the blastopore.
