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Today I learned about conjoined twins. I had always heard that conjoined twins form when the embryo doesn't separate fully, but apparently there is also a theory that the twins form after the embryo has already separated; the cells seek out like cells to join to, and sometimes instead of finding cells in the same body, they find cells in the other twin and the two become fused. I know this theory is not as accepted as the first, but I don't know if it's like plausible or if anybody halfway intelligent dismisses it entirely. To me, though, it doesn't seem that likely. I mean, why wouldn't you ever have conjoined twins where, say, the head of one twin is growing out the back of the other? Or what prevents non-identical twins from becoming fused in the same way? Maybe the cells that find each other have to be almost identical--they have to come from the same body part or have the same DNA or whatever. I don't know. I'm not a scientist.