Chestburster (from ALIEN)

The incubating embryo takes on some of the host's DNA or traits, such as bipedalism, quadrupedalism, possessing the mandibles of a Predator, and other structural changes that enable adaptation to its new environment. According to Weyland-Yutani medical scientists in Aliens: Colonial Marines, the chestburster will draw nutrients from the host's body in order to develop a placenta as it grows, attaching itself to several major organs in the process. The placenta has cancerous qualities, such that even if the embryo were removed surgically, the placenta would simply cause the affected organs to shut down, resulting in death; the only exceptions to this are from human-xenomorph hybrid hosts like the cloned Ripley 8, who survived an extraction procedure without issue.

Over the course of one to 24 hours—indeterminable in some cases, and sometimes up to a week, in the case of some queens—the embryo develops into a chestburster, at which point, it emerges, violently and fatally ripping open the chest of the host.When a chestburster erupts from the body of a human host, it is less than 30 centimetres (12 in) tall, although the embryo can vary in size from a guinea pig to a large dog depending on the size and species of the host. Its appearance and adaptive characteristics are also determined by the host. Typically, its first instinct upon emerging is to flee and hide until full maturation, as well as find a source of nutrition. However, it soon undergoes a dramatic growth spurt, reaching adult size in a matter of hours; in Alien, the chestburster had grown to 2 metres (6.6 ft) in height by the time the Nostromo crew located it again. The chestburster is shown to have molted before reaching maturity. In Aliens vs. Predator: Requiem, Alien warriors who are still growing are shown, displaying shed skin. In the unrated cut, the Predalien is shown wiping off its final molted skin at the film's start.