The first stage of the alien's lifecycle consists of an egg. The eggs are complex organisms in and of themselves. They are responsible for maintaining life support for the larva for an indefinite amount of time, and must recognize a potential host and distinguish it from valid members of the nest. Eggs are initially created inside a queen xenomorphand enter the world via slimy tube to stand on their own (indicating that there is a definite "up" side and "down" side to the egg). The egg itself is a leathery object approximately three feet tall.

The egg can survive for an undetermined length of time in various environments. The egg is translucent , is flammable, and unarmored. The egg detects when a host is proximal by sound and/or movement. As this happens, some sort of liquid seems to drip upward, and small bumps appear as the egg comes to live, awakened by the chance to continue it's lifecycle. The petals(?) at the top of the egg open, preparing for the next stage as the face-hugger.
We first encounter these eggs in 'Alien' when Kane is lowered into the depths of an unknown space craft that has been sitting on the surface on planet LV-426, later to become Archeron. This bio-organic ship appears to have crashed onto the planet and begun to send what was first thought of as a distress signal, but later determined by Ripley as a warning signal. Kane enters what could be called a huge storage room that has many large beds covered by a list blue mist. Underneath the mist, many eggs can be seen.

We can see near the end of 'Aliens' that eggs are very flammable, and torch right up when Ripley flames them. In this same scene we see that the egg sac creating the eggs is full of a lot of liquid, which also catches on fire. This flammability could be due to the acid that the xenomorph species posesses.

Although we know that the queen laid the eggs, originally eggs were constructed by different means. According to a scene that was cut from 'Alien' these eggs could also be "constructed" by a regular alien "infecting" an organism however, this concept was not supported (nor denied) in 'Aliens' and 'Alien3'. This was the concept originally used with Brett and Dallas in 'Alien'. It is important to note that this method was the originally intended method of the designer of the Alien, H.R. Giger.

One more thing to notice is that the egg seen in Alien3 was different looking. It was smaller and more bulbous, unlike than the ones seen in 'Alien' and 'Aliens'. We already know the alien in 'Alien3' was different. This difference might have been due to the egg itself, the host or even the environment.