The first process is gastrulation (1) which is the movement of cells from the blastula surface to the interior of the embryo. Gastrulation rearranges the cells of the blastula into a three-layered embryo called a gastrula (1). The three layers produced by gastrulation are called embryonic germ layers (1) – they are known as the ectoderm (1) (which forms the outer layer), the endoderm (1) (which lines the digestive tract) and the mesoderm (1) (which partly fills the space between the endoderm and the ectoderm (1). Each germ layer contributes to specific structures (1) in the adult animal. The newly formed cavity is called the archenteron (1) which opens through the blastopore (1) which will become the anus in some animals. 

A blastocyst (1) is the human equivalent of the blastula. The inner cell mass is a cluster of cells at one end of the blastocyst. The trophoblast (1) is the outer epithelial layer of the blastocyst and does not contribute to the embryo, but instead initiates implantation (1).   

Following implantation, the trophoblast continues to expand and a set of extra-embryonic membranes is formed. These enclose specialized structure outsides of the embryo. Gastrulation involves the inward movement (1) from the epiblast (1) through a primitive streak (1). Humans form four extra-embryonic membranes known as the chorion (which functions in gas exchange) (1), allantois (disposes of waste products and contributes to gas exchange) (1), amnion (encloses the amniotic fluid) (1) and yolk sac (encloses the yolk) (1).