It is possible for an early zygote to fracture into two separate zygotes, leading to the development of identical (monozygotic) twins. It is also possible for incomplete fractures to occur or for multiple zygotes to fuse together, leading to conjoined twins, or individuals with some structures of a sibling—as with two headed reptiles.
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Totipotency refers to the ability of these cells to develop into all possible cell types over time. It is possible for any combination of these developments to happen, though typically the most dramatic versions are not able to survive to birth or long past it. Rarely, however, individuals are seen with twin parts, as is the case with this turtle’s second head.