The Egg sac, also known as an ovipositor, is the part of a Xenomorph Queen that creates and then lays Eggs. While a Queen is perfectly capable of surviving without an Egg sac, she cannot produce more Eggs in this state.
A Queen's Egg sac is enormous, easily dwarfing the size of her body. Typically stretching for many tens of feet, the structure is analogous to the large egg-producing abdomen of queen termites. Eggs are produced inside the Egg sac constantly in a fashion similar to a mass production line, first forming at the base of the sac, where it connects to the Queen herself, and developing as they move towards the rear. Finally, the developed Eggs are laid by an ovipositor at the Egg sac's tip.
Owing to its immense size and weight, a Queen is immobile once her Egg sac has developed; the sac itself is typically suspended from the roof of the Hive by resin "straps". As such, the Queen is dependant on her brood for defense and care. Other Xenomorphs also tend to the Eggs once they are laid, moving them from beneath the ovipositor to "nursery" areas within the Hive. In an emergency, the Queen can separate from her Egg sac to allow her to move, although this process is apparently quite painful for her. While she cannot subsequently reattach the discarded sac, she can apparently grow a new one fairly quickly. Notably, despite the Egg sac making up a huge proportion of the Queen's overall mass, destroying it is not fatal to the Queen herself.
- In Alien Resurrection, the Cloned Queen sheds her Egg sac and replaces it with a womb-like appendage that she uses to give live birth to a new kind of Alien, the Newborn. This alternate reproduction method was brought about as a result of genetic cross-contamination resulting from the cloning process used to create the Queen's original host, Ripley 8.