The process by which gastrula stage is formed from blastula is called gastrulation. In this process, the cells change rapidly and are arranged in two layers. The outer one is called ectoderm and the inner one is called endoderm. Later mesoderm is formed from ectoderm. A fluid-filled cavity forms inside the gastrula. It is called archenteron. The opening through which the archenteron is released is called the blastopore.