Embryology This is the study of growth, fertilization, division or differential of the zygote into embryo of early development of living beings before the attainment of structure and size of the offspring is called embryology. Embryology is a science that is about the development of an embryo from the fertilization of the ovum to the fetus stage. After cleavage, the dividing cells which called  morula, becomes a hollow ball, called blastula, that develops a hole or pore at one end.

In bilateral animals, the blastula that is a hollow ball develops in one of two ways which divides the whole animal kingdom into two halves becomes the mouth of the animal, it is a protostome. The protostomes include most invertebrate animals, such as worms, insects, or molluscs, the blastula changes into a more differentiated structure known as gastrula.