Produced by Alien Queens using their ovipositor, Eggs are fully developed upon being laid, therefore not requiring the need to fertilize, although a hot environment is preferable for optimum incubation. Eggs generally appear as ellipsoidal, leathery objects around two-three feet high with a four-lobed opening at the top. Rather than the Egg itself, the Xenomorphs' true reproductive method lies through the small parasitoid housed within it, the Facehugger. A single Egg houses a single Facehugger, putting the creature in a state of suspended animation. The Alien Egg possesses incredible longevity and is able to sustain both itself and the Facehugger for a number of years, possibly indefinitely with no external nutrients or assistance. Once a potential host approaches, the Egg's lobes would begin to unfold into four "petals", and the Facehugger becomes active, launching itself from the Egg and attaches itself to the potential host. Once the Facehugger had exited the Egg, it is no longer of further use, thus have fulfilled its purpose in the Aliens' life cycle.
Appearances in other Media
Dark Horse Comics
The following information follows the continuity from Dark Horse Comics' Aliens comics and all related media, therefore unrelated to the primary continuity.
Alien Eggs appeared in Dark Horse Comics' Aliens comic book series, set on an alternate continuity following the events of Aliens.
Behind the Scenes
The Egg first appeared in the 1979 film Alien. During the scene where Kane was exploring the derelict, a fiberglass Egg was used so that Kane's actor, John Hurt, could shine his light on it and see movement inside, which was provided by director Ridley Scott fluttering his hands inside the Egg while wearing rubber gloves. The top of the Egg was hydraulic, whereas it's interior was composed of caul fat, which is the lining of a cow's stomach. In addition, the quick shot of the Facehugger erupting from the Egg was done with sheep's intestine.
H.R. Giger initially designed the Eggs with a much more obvious vaginal appearance, complete with an "inner and outer vulva". The producers complained that Catholic countries would ban the film if the allusion was too strong, so Giger doubled the lobes to four so that, in his words, "seen from above, they would form the cross that people in Catholic countries are so fond of looking at".
Alien had a deleted scene where it would've introduced an alternate method of the Alien to produce Eggs. During the film's climax, Ripley would have discovered both Captain Dallas and Brett cocooned in the Nostromo's hold. They were on the verge of being morphed into new Alien Eggs. Captain Dallas, who had regain consciousness, begged Ripley to kill him so that he could be spared from the agonizing process. Ripley complied, torching both him and Brett with her flamethrower. Ridley Scott decided to cut the scene entirely from the theatrical release as he felt that it would have slowed down the film's final act. The scene was included in the novelization by Alan Dean Foster, and subsequently the Director's Cut of Alien released in 2003.