In development of human, the egg cell (from a women) and the sperm cell (from a man) fuse together to form a single cell called the zygote.
The zygote divides multiple times to form 216 different cell types and these are the precursor cells for formation of the entire two to three trillion cells that eventually forms the entire human body.
The zygote which is formed and the first three cell divisions are capable of forming the entire human body.
These formed cells are called as totipotent stem cells.
Identical twins are formed when the dividing cell mass splits apart at this stage, which leads to genetically identical embryos.