Blastopore

Gastrulation
Gastrulation occurs when a blastula, made up of one layer, folds inward and enlarges to create a gastrula. This diagram is color-coded: ectoderm, blue; endoderm, green; blastocoel (the yolk sack), yellow; and archenteron (the gut), purple.
Anatomical terminology []

Gastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar ("three-layered") structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.

Gastrulation takes place after cleavage and the formation of the blastula. Gastrulation is followed by organogenesis, when individual organs develop within the newly formed germ layers. Each layer gives rise to specific tissues and organs in the developing embryo. The ectoderm gives rise to epidermis, and to the neural crest and other tissues that will later form the nervous system. The mesoderm is found between the ectoderm and the endoderm and gives rise to somites, which form muscle; the cartilage of the ribs and vertebrae; the dermis, the , blood and blood vessels, bone, and connective tissue. The endoderm gives rise to the epithelium of the digestive system and respiratory system, and organs associated with the digestive system, such as the liver and pancreas. Following gastrulation, cells in the body are either organized into sheets of connected cells (as in epithelia), or as a mesh of isolated cells, such as mesenchyme.