The Egg Sack, roughly the size of a bird's egg, is able to sense the proximity of potential hosts, spewing a cloud of microscopic, insect-like spores called Motes into the air. The motes are able to group together, instinctively invading the host's body through open orifices, ultimately resulting in the gestation of a Bloodburster.
The Egg sack is generally considered to be the first stage in the life cycle of the Neomorph species, and closely resembles the Clostridio tetani and Euniceral Dane Eforedium pathogenic fungi native to Planet 4.
- The design of the Egg Sack resembles a smaller version of H.R Giger's Egg Silo, a design originally intended to be used in the film Alien.
- The Egg sack closely resembles and functions as Lycopedron pyriforme puffball fungi.
- In the novelization of the film, the Egg sacks are much smaller in appearance, resembling mould.