During fertilization, the sperm and egg fuse to form a new diploid cell called a zygote.
The zygote’s cells divide, forming a new, multicellular cluster called a blastocyst, which travels to the uterus where it implants itself on the inner surface of the uterine wall.
A few hours after syngamy, when the sperm and egg have fused into a zygote, the new diploid cell undergoes a process called cleavage, dividing into a new pair of cells called blastomeres.
The blastomeres keep splitting, becoming a loose clump of four cells, then eight cells, and finally a more structured, mulberry-shaped 16-cell cluster called a morula, with inner and outer cell masses.
The morula’s cells are held in a vaguely spherical arrangement by the zona pellucida.
The morula gradually develops an outer cell mass of trophoblast cells and an interior cell cluster with a fluid-filled cavity at the core, which is called the blastocoel.
As soon as the blastocoel forms, the morula is no more: it’s now a water balloon-shaped arrangement of cells called a blastocyst.
The cells making up the inner cell wall of the blastocyst are collectively called the embryoblast, because they will go on to form the fetus.
The embryoblast cells cluster together at one end of the blastocyst in an area called the embryonic pole.
Meanwhile, the trophoblast cells flatten out into a barrier around the blastocyst called the epithelial wall.
Fully-formed, the blastocyst hatches from the zona pellucida around the end of day four, and is now ready to attach itself to the wall of the uterus.