Describe the process of gastrulation. From which parts of the developing gastrula will later-embryonic tissues develop?
The developing blastula is characterized by the fact that the cells of one hemisphere (the vegetal hemisphere) are generally larger than those of the opposite hemisphere (the animal hemisphere). Gastrulation begins when an invagination at the pole of the vegetal hemisphere occurs. The cells proceed to push into the blastocoel until the invaginating cells lie in a layer right up against the outer layer of cells. The new cavity formed by invagination is called the archenteron; it will become the digestive tract in the embryo. The opening to the cavity is called the blastopore; it will become the anus in the developing fetus. The two layers of cells of the gastrula will become the three primary germ layers in the embryo.
