Left: Contemporary photomanipulated tabloid image of "Cloudy Day", a three-legged stripper. Right: Cabinet card by Charles Eisenmann of unknown child with parasitic twin (Mitchell).
Types of Parasitic Twins
Epigastric parasites. Jack Hunter calls these "heteradelphians" (literally 'different sibling'). This category describes the phenomenon of an incomplete twin, usually consisting of a diminutive body attached at the epigastrum (lower abdomen). Some of these twins have a rudimentary head imbedded in the autosite's (host twin's) abdominal cavity, and the growth of this structure can cause complications with organ growth. In the case of Betty Lou Williams, this is probably the cause of her death. All of these twins are acephalic-acardiac, meaning they are both without a brain and without a heart, and completely dependent on the autosite.
Dipygus ('double buttocks') or pygomelia ('limbs attached to the buttocks'). This describes duplication of the lower extremities. In complete dipygus, two small pelves are situated side-by side and the person has active control over all four legs. In Myrtle Corbin, both sets of sexual organs were also fully functional. Though her inner legs were small and atrophied due to lack of use, she was able to move them independently. Abdul-Aziz Rainloun, born in South Africa in 1992, also had complete dipygus, but two of his four legs were surgically removed when he was 3 years old. Dipygus parasiticus cases have imperfectly developed legs attached to the pelvis or lower spine and may have passive control over these limbs, but cannot move them individually. The twin, in this case, may have extra hands, feet, breasts, etc. as well.
Craniopagus parasites. The most famous example of this case was the Two-Headed Boy of Bengal (since I don't know his real name, I haven't made a page for him). This child had a second, well-developed head attached upside-down at the top of his cranium. His configuration resembled that of vertical craniopagus twins, except that the neck of the second head ended in a small stump of a neck. When he made facial expressions, the features of the second head also moved; when he nursed, the second mouth salivated. Sadly, the child died from a cobra bite at the age of 4 years. His body was exhumed by Everard Home, the apprentice of the famous surgeon John Hunter. Upon autopsy, the neck stump of the parasitic head was shown to contain fragments of bone and tiny vestiges of a heart and lungs. The skull still resides in the Hunterian Museum in London.
A more recent case of a craniopagus parasite is Rebeca Martinez from the Dominican Republic. Born in December 2003, Rebeca had a second head very similar in structure to that of the Two-Headed Boy. Its facial features were badly deformed and some vestigial limb buds were attached to the neck stump. The second head also contained a partial brain and scans showed some low-level brain function belonging to the parasitic twin. Tragically, Rebeca Martinez bled to death during an operation to remove the parasitic twin, which doctors predicted would have interfered with her mental development if left intact.
Fetus in fetu. One fetus is encapsulated in a cyst inside the body, usually in the abdominal cavity, of its twin. Gould & Pyle's Anomalies and Curiosities of Medicine (1896) presents many cases of fetus-in-fetu, some of which were not detected until the "host" twin began complaining of abdominal pains in adulthood.
There is another condition frequently confused for fetus-in-fetu, known as a dermoid cyst. These cysts can become very large and are lined with actual skin tissue that can grow hair, sweat, and secrete sebum (skin oil). Dermoid cysts have also been found to contain bone fragments, teeth, and even partial faces and tongues. They are often filled with a waxy substance consisting of shed skin cells and sebum, which usually slough off of our skin, but have nowhere to go inside the cyst and thus are compacted into solid matter.
Diprosopus and Sonic Hedgehog
Since several hundred thousand (possibly several million) separate proteins are involved in the making of a complex life form, biologists often give them whimsical names in order to keep them straight. "Sonic hedgehog" was the name given in the 1980s to the protein, found only in embryos, which is responsible for establishing the body's midline. A sonic hedgehog deficiency was found to be the cause of cyclopia - a gruesome, fatal birth defect in which both eyes are contained within the same socket, giving the appearance of the legendary Greek Cyclops. Cyclopia is a symptom of holoprosencephaly, literally "whole forebrain", meaning that the brain is not divided into hemispheres like a normal brain, but is rather a single lump of tissue. Since the hemispheric division of the brain - produced by the sonic hedgehog protein - is the basis for the midline, embryos without divided brains are confused, so to speak, about how far apart their eyes need to be. Sonic hedgehog is the reason most of us have two eye sockets, set a reasonable distance apart, with a symmetrical nose in between them. A "cyclops" has only one eye socket and no nose.
Though less understood than cyclopia, sirenomelia (literally "mermaid limbs" - a fatal fusion of the lower limbs into a single, flipper-like extremity) is believed to be another defect caused by sonic hedgehog deficiency.
What, then, is the outcome when too much sonic hedgehog is introduced into a developing embryo? Scientists have produced this scenario artificially by dousing chicken embryos with an abnormally large dose of the protein. The result is that many of the chicks are born with their eyes very far apart. Some even have two beaks. This would suggest that duplication of the facial features may in fact be caused by an overdose of sonic hedgehog - not by incomplete twinning, as is frequently believed. Armand Marie LeRoi (author of Mutants) maintains that Ditto, a two-faced pig who appears in a California sideshow, is an example of how sonic hedgehog can be the cause of diprosopus.
(Since writing this article, I discovered that Ditto the pig died of aspiration pneumonia at around 18 months of age. Apparently one of his faces was eating while the other was breathing, causing food to enter his lungs. R.I.P. Ditto.)