There's also some confusion over the origins of the exoparasitic twins—the ones who materialize on the surface of their siblings' skin as extraneous body parts. Embryologists don't have much data to work with, as such cases occur just once in every million live births. But most believe they come about in more or less the same way as regular conjoined twins. (The incidence of those is closer to 1 in 200,000.) According to the most popular theory, a single embryoblast—a mass of cells that later give rise to the differentiated structures of the fetus—spontaneously divides about two weeks after fertilization, resulting in two connected centers of growth. That's exactly how identical twins form in the womb, except in that case the fission occurs a few days earlier. When an embryoblast splits at around Day 9, the two halves tend to form into discrete, functional units—like the Weasley brothers in Harry Potter. But if the process gets held up until Days 13 or 14, then you may end up with a " double monster." (That was a technical term, by the way, as of 1961.)
So conjoined and parasitic twins may represent two points on a spectrum of identical twinning gone wrong. At one end are the symmetrical siblings, like Krista and Tatiana Hogan—the Canadian toddlers whose bodies are fused at the brain. At the other end, one sibling has deteriorated in the womb, leaving little more than a couple of feet, a thorax with nipples, or a stray set of genitals. More developed parasites may have some viscera and bones, even whole organs, but only rarely is there any semblance of a heart or brain.
If parasitic twins are conjoined twins that happen to be very asymmetrical, then you'd expect to find certain similarities between these categories. Indeed, Spencer notes that both types tend to be connected at one of eight standard locations. Thus we have the craniopagus twins and parasites attached at the cranial vault, like Krista and Tatiana, or the Indian man we met earlier; the omphalopagus or thoracopagus variants, who stem from the thoracic region of their siblings, essentially growing out of the chest; the cephalopagus twins, connected at the mouth, ears or nose; the ischiopagus, originating in the lower abdomen; the pygopagus, joined at the sacrum, perhaps with a superfluous arm or leg that looks like a tail; the rachipagus, unfolding from the vertebral column; and parapagus, popping out from somewhere on the pelvis.
There are a few cases, however, where the model seems to falter. If identical twins, conjoined twins, and parasitic twins all start the same way—with the splitting of a single fertilized egg—then the pairs of each kind should have a shared genetic profile. When researchers have performed this analysis on parasites and autosites, the answer almost always comes up as expected: Their DNA is the same. But there are some outliers. In Santo Domingo, for example, a parasite and its host were found to have an "allelic discordance" suggesting they were a rare type of fraternal twins. There's also the case of a little girl in New Delhi who arrived in the world with an unexpected visitor attached to her back, a "lumbosacral parasitic rachipagus twin." According to Rajiv Chadha and his team from the Lady Hardinge Medical College,
The mass had a bony-hard feel underneath the soft tissues. Three globular structures resembling aborted digits were visible, along with a small mass covered with skin having the pigmentation and rugosity of the scrotum.
If you missed the important point of (on?) this strange mass on the baby's back, it's all in the scrotum. Again, the theory holds that both conjoined twins and parasitic twins are monozygotic, which means, of course, of the same biological sex. So a parasitic scrotum attached to a female autosite rattles the conventional wisdom.
Some scientists believe these flukes result from a different process in the womb. Instead of one embryoblast splitting late and dividing just part of the way, it could be that two embryoblasts come together unexpectedly. (In theory, this merging of fraternal twins could produce any of the abnormalities reviewed earlier.) The New Delhi physicians didn't have the resources to conduct genetic testing on their ostensibly mixed-gender pair, so it's certainly possible that the parasite's "scrotum" was in fact something else entirely. Still, the "fission vs. fusion" battle rages on in the world of abnormal embryology.