The process of embryogenesis begins when an egg or ovum is fertilized by a sperm cell to form a zygote.
This zygote then undergoes mitotic division, a process that does not result in any significant growth but creates a multicellular cluster called a blastula. This process is referred to as cleavage. The blastula then develops a pore or a hole at one end, called a blastopore.
Initially, at least four cell divisions occur, giving rise to sixteen cells, collectively referred to as the morula. From there, the cells that form during cleavage up to the blastula stage, are called blastomeres. The ball of cells is referred to as a blastula, once cleavage has produced around 100 cells. The blastula is made up of a hollow spherical layer of cells, referred to as the blastoderm which surrounds a yolk or fluid-filled space called the blastocele or blastocoel.
In most mammals, including humans, the structure formed next is the blastocyst, a mass of inner cells that are distinct from the blastula. The next stage of embryogenesis is formation of the gastrula.
Before gastrula formation can occur, the cells of the trophoblast are differentiated into two layers – one is an outer layer of protoplasm that contains the nuclei called the syncytium and the other is an inner layer of well-defined cells called the Layer of Langhans. The trophoblast cells do not help to form the embryo proper but from the ectoderm of the chorion and contribute to the development of the placenta.
Inside the inner cell mass is a layer of flattened cells called the entoderm. This forms a small sac called the yolk sac. Between the remaining cells, spaces appear which merge and expand, eventually creating the amniotic cavity. Forming the base of this cavity is the embryonic disk, which first becomes oval and then pear shaped. At this stage, the blastoderm contains three layers germ cell layers and the embryo is called the gastrula.
All of the body’s organs and tissues are eventually derived form these three layers. From the innermost layer or endoderm, the digestive organs, lungs and bladder develop; the skeleton, blood vessels and muscles are derived from the middle layer or mesoderm and the outer layer or ectoderm gives rise to the nervous system, skin and hair.