Describe the stages of cellular development prior to tissue stages.

Following fertilization, the single-celled zygote undergoes cleavage, where it is then divided into numerous tiny cells. After 3 to 4 days of fertilization, the cleavage produces a 16 celled embryonic stage known as morula, which is contained within the zona pellucida.

After about 4-5 days of fertilization, cleavage produces a sphere mass of cells known as blastomeres, which is covered by a layer known as trophoblast and has a fluid-filled cavity in the center, known as blastocoel. The blastomeres then migrate inward, changing position and undergoing rearrangements. This is known as gastrulation.

Gastrulation then causes the development of three germ layers: ectoderma, endoderma, and mesoderma. The interaction of cells and the rearrangement of tissues and organs occurs only after the formation of germ layers.