3. Parasitic Twin (Fetus in Fetus)
The independent twin is called the autosite.
It is a exaggerated case of siamese twins. Two twins don´t get to separate completely when they are zygotes and they are united by some zone. One of these twins grows healthy whereas the other degenerates, remaining inside the healthy twin and depending completely on him. It is unkown why these twins don´t separate correctly. Unlike conjoined twins, one ceases development during gestation and is vestigial to a mostly fully-formed, otherwise healthy individual twin. The undeveloped twin is defined as parasitic, rather than conjoined, because it is incompletely formed or wholly dependent on the body functions of the complete fetus.