This is thought to be the most likely reason why conjoined twins occur, and could possibly play a role in the development of craniopagus parasiticus.One hypothesis is that craniopagus parasiticus starts with the development of two fetuses from a single zygote that fail to separate at the head region around the second week of gestation. Some believe, however, that it occurs later in development, around the fourth week of gestation, at which time the two embryos fuse together near the anterior open neuropore.Another hypothesis is that there is joining of the somatic and placental vascular system of the twins, as well as a degeneration of the umbilical cord of the parasitic twin. This suggests that craniopagus parasiticus develops due to the lack of blood supply to one of the twins. Diagnosis
Related conditions
In addition to craniopagus parasiticus, a few other conditions involve a childs, or animals, birth with two heads or faces. Dicephalic parapagus
Dicephalic parapagus is a condition where two heads are side by side on a single torso, with varying degrees of twinning of organs and structures within the torso. Diprosopus
Diprosopus occurs when a single body and neck are present, but there is a duplication of the structures of the face. This is different from craniopagus parasiticus in that there is only one head, although there is a duplication of the craniofacial features. Diprosopus can range from having two fully formed faces to just a duplication of the nose or eyes. Cats with the condition are referred to as Janus cats.
See also
Pasqual Piñón, a sideshow performer billed as the Two-headed Mexican
Janus, ancient Roman god depicted with two faces
Edward Mordrake, an urban legend with a similar trait
Futakuchi-onna, Japanese yōkai with a similar trait
Tomie Kawakami, a titular character of Tomie whom possess with the two-headed appearance

References
Bibliography
External links
Image File of craniopagus parasiticus