Woodcut of a "man from whose belly another man issued" from Des Monstres et prodiges by Ambroise Par矇, 1573. A parasitic twin (also known as an asymmetrical or unequal conjoined twin) is the result of the processes that produce vanishing twins and conjoined twins, and may represent a continuum between the two. Parasitic twins occur when a twin embryo begins developing in utero, but the pair does not fully separate, and one embryo maintains dominant development at the expense of the other. 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. The independent twin is called the autosite. Des Monstres is filled with unsubstantiated accounts of sea devils, marine sows, and monstrous animals with human faces. With its extensive discussion of reproduction and illustrations of birth defects, the book invited accusations of pornography.