Embryology is the study of the organism's "pre-birth" development, starting with the zygote stage. A zygote is a fertilized_egg. The first event is the division of the zygote into two equally sized cells termed blastomeres. Each blastomere divides until a solid mass of cells (morula) develops.
The sequence of divisions that leads from the single-celled zygote to the many-celled morula is called the cleavage_pathway. After the morula forms, some cells at the center of the ball start to produce a fluid. As the fluid accumulates, it exerts pressure that forces the cells at the center of the morula to move outward. This is how the next stage, which is termed the blastula (hollow-ball) stage, forms.
The next event is the formation of the gastrula stage. The gastrula stage begins with the invagination of the blastula. At the center of the invagination, a group of cells known as the inner_cell_mass forms. These cells will develop into the fetus and some of its supporting membranes. The remainder of the gastrula will also form supporting membranes.
This micrograph shows the various stages of embryonic development that occur between the formation of the zygote and the development of the gastrula stage.