A fetus in fetu can be considered alive, but only in the sense that its component tissues have not yet died or been eliminated. Thus, the life of a fetus in fetu is inherently limited to that of an invasive tumor. In principle, its cells must have some degree of normal metabolic activity to have remained viable. However, without the gestational conditions attainable (so far) only in utero with the amnion and placenta, a fetus in fetu can develop into, at best, an especially well-differentiated teratoma; or, at worst, a high-grade metastatic teratocarcinoma. In terms of physical maturation, its organs have a working blood supply from the host, but all cases of fetus in fetu present critical defects, such as no functional brain, heart, lungs, gastrointestinal tract, or urinary tract. Accordingly, while a fetus in fetu can share select morphological features with a normal fetus, it has no prospect of any life outside of the host twin. Moreover, it poses clear threats to the life of the host twin on whom its own life depends.http://www.jmedicalcasereports.com/content/2/1/2
There are two main theories about the development of fetus in fetu; one simple, the other complex.
Fetus in fetu may be a parasitic twin fetus growing within its host twin. Very early in a monozygotic twin pregnancy, in which both fetuses share a common placenta, one fetus wraps around and envelops the other. The enveloped twin becomes a parasite, in that its survival depends on the survival of its host twin, by drawing on the host twin's blood supply. The parasitic twin is anencephalic (without a brain) and lacks some internal organs, and as such is almost always unable to survive on its own.
Sometimes, however, the host twin survives and is delivered. The parasitic twin grows so large that it starts to harm the host, at which point doctors usually intervene. The condition causes the host to look pregnant, and can occur in both males and females.
Cases of fetus in fetu sometimes attract worldwide media attention. These cases are a small minority of the known cases and rarely overlap with cases reported in the medical literature, but they are widely accessible.
A more complex development of the idea takes place in the animated television series The Venture Bros., where the main character Dr. Thaddeus Venture has a fetus in fetu (Jonas Venture Jr.) growing inside him for forty-three years. After Jonas Venture Jr. is surgically removed in the first season finale, he initially attempts to kill his brother in retaliation for being engulfed, though later the brothers make peace and the fetus in fetu becomes a jovial genius.
The concept is also the central theme of the novel Passenger by Billy Cowie (published by Old Street Publishing 2008) where the main character Milan discovers his sister Roma is embedded in himself and spends the remainder of the book attempting to communicate with her.
In both the manga and anime of Black Jack, the character of Pinoko is a parasitic twin who was removed and given a new life within an artificial body.