Craniopagus twins conjoined twins develop inutero remains unclear. Embryologists have traditionally attributed identical twinning as "splitting or fission" of either the inner cell mass of pleuripotential cells or early embryonic disc at 13–14 days of gestation just before the primitive streak. Some theorists suggested that conjoined twins develop as a result of the failed fusion of a single fertilized ovum. However a new hypothesis suggests that cranial fusion occurs between two separate embryos before the end of the 4th week of gestation. This can happen because the cranial neuropore is still open, which is responsible for the ultimate fusion and formation of
Trigger Twins Secret Society of Super Villains. As part of an army, they are sent to destroy the city of Metropolis. Another army of superheroes face them. During what is called "Battle of Metropolis", the twins are shot down in the street by a group of vigilantes that include the current Vigilante and Wild Dog who were perched on a nearby rooftop. This battle takes place in "Infinite Crisis" #7. During the "Blackest Night" crossover, the Trigger Twins are among the many deceased villains that receive a black power ring, reanimating them into Black Lanterns. The Earth 18 version of Trigger Twins