Gastrula is the spherical embryonic structure consisting of 3 germ layers. Blastula is converted into gastrula through gastrulation. The movement of blastomeres to defined places in embryo to create a triploblastic structure is called Morphogenetic movements. They include invagination of endoderm, evolution of pharyngeal endoderm and chordamesoderm and epiboly of ectoderm.

The outer layer is ectoderm from which epidermis and nervous system develop. Inner endoderm endoderm the primitive gut or archenteron. Ectoderm and endoderm are epithelial while mesoderm is sometimes epithelial and in some cases, it is mesenchymal (i.e., loosely arranged cells with much intracellular materials). In gastrula, walls and floors of archenteron are composed of endoderm, but its roof is formed of chordamesoderm. The opening of gastrula is called as a blastopore.

A ventral section of gastrula shows following structures. The notochord and neural plates are well differentiated and lies at the dorsal side. Blastocoel is diminished due to the development of archenteron. Epidermis lies at ventral side and archenteron towards the dorsal side. Some of the yolk cells are not enclosed until a relatively late stage in the development and protrude through blastopore as yolk plug. The mesoderm increases relatively in comparison to blastula.