Embryonic tissue development in animals.
Cleavage of the zygote by successive mitotic divisions produces a blastula, a hollow ball of cells.
Then the embryo folds in at the blastopore toward the blastocoel (cavity), forming a gastrula with germ layers of embryonic tissues: the ectoderm and the endoderm; some animals have an additional layer, mesoderm.
Germ layer development