Chestburster

The Chestburster is the immature infant form of the Xenomorph which emerges forcibly from the chest of its host, killing it. The chestburster is similar in form to the fully grown alien but far smaller and pale in color.

It's egg is laid in the host's bronchial tubes by a Facehugger, where it hatches. It then burrows for the host's aorta (major artery in the heart), where it feeds on absorbed nutrients and white blood cells. After about an hour or two, it lets go, and begins swimming about in the host's body, causing mild discomfort. Then, it finds the sternum, and starts chewing. This causes the victim geat pain, shock, and blood loss. Then, true to it's namesake, it pops out, and finds a nursery or chicken coop, or, less messy, a food locker, something with a large amount of helpless meat. It's carapace has yet to harden, and is a dull grey fleshy hide. Depending on the host, it will grow to become a Drone (human, black carapace (standard Xenomorph)), Predalien (Yautja, yellowish-green partial hide), or Runner (any given quadruped, red hide). Genetic alteration causes it to split into identical twins after escape from a human or quadruped (AVP Extinction).

An embryo implanted in a pregnant woman will feed directly on the baby, and will erupt from the stomach of the woman. (Avp:R)

Behind the scenes
The chestburster was designed by Alien special effects master Roger Dicken. Giger had produced a model of a chestburster that resembled a plucked chicken and was far too large to fit inside a ribcage. Much to Giger's chagrin, his model reduced the production team to fits of laughter on sight, Scott drafted a series of alternate designs for the chestburster based on the philosophy of working "back [from the adult] to the child" and ultimately produced "something phallic," so Dicken was given the task to design it. Giger stated the finished form first resembled a dinosaur but the arms and legs were removed.

The chestburster in Alien 3 was far more developed, possessing complete limbs. The Queen chestburster depicted in Alien 3 has all four arms.