What is gastrulation? How during gastrulation are the first two germ layers formed? What are these germ layers?
Gastrulation is the process by which a portion of the blastula wall undergoes invagination inside the blastocele, forming a tube known as archenteron (primitive intestine). The cells of the inner side of the tube type the endoderm (germ layer) and the cells of the outer side form the ectoderm (another germ layer). It is the starting of the tissue differentiation in embryonic development.