The eggs are made mostly by a Queen. A ritual us Yautja have had for the last few hundred years is the seeding of planets with eggs, in preparation for the Rite of Passage. The eggs are about three feet tall, and kept in a large chamber of the hive with the Queen.
Inside the egg is a creature (with eight legs and a strong prehensile tale) which serves as carrier for the embryo of a new Xenomorph. It latches onto a victim's face, then implants the embryo down the throat. The embryo grows inside the host, until it is big enough to survive on its own. During this time, it gathers information from the host's DNA to better adapt to the host's environment. It uses the information to alter its growth, and inherits certain key traits that the host possessed. This way, the deadlier the host, all the deadlier the Kainde Amedha.
Once deposited by the facehugger the embryo grows in the chest cavity. Because of the apparent dietary, respiratory, and circulatory needs of the Xenomorph this placement in the host's chest is in no way related to specific biological needs (such as highly oxygenated blood as supplied from the host's heart). It would seem that the only reason for the placement of the embryo in the chest cavity is for protection. Behind the sternum the embryo is protected from heavy jarring or damaging blows. Unfortunately this placement also means an extremely painful death for the host, as the embryo has to repeatedly "punch" the host's sternum until it has broken free. The amount of time needed to reach full development from internal embryo to "burster" is only 2 to 4 hours from the time the Facehugger has "fallen" from the host. Initially a Queen embryo would remain in the host for upwards of 24 hours from implantation.
Adult grown Alien Drone
This, the final stage of the Xenomorph's development, is undoubtedly the most hostile stage -- both structurally and in temperament. It stands about 2 meters tall, is generally bipedal, possesses a strong tail that extends to a length equal to that of the body, and has an impressive physical strength considering its body size. Perhaps the most interesting, and disturbing, feature of the adult Xenomorph are the vicious, and incredibly powerful, secondary jaws. These jaws are part of a modified tongue, and are used in an offensive, rather than defensive, manner. The tongue is approximately 16 inches in length with a striking distance of up to 12 inches from the creature's mouth. The organ is, as are mammalian tongues, comprised entirely of muscle.
Alien Queen
The Xenomorph Queen is approximately four and one half meters tall, possesses an extremely powerful tail that is equal in length to the Xenomorph's height, a cranial crown that is flattened (in comparison to the adult Xenomorph) and extends approximately two meters beyond the back of the head, and possesses a second set of arms (giving the Xenomorph a total of six limbs) that are approximately one third the length of the primary arms. Her primary concern is the lives of future young, whom she will protect at the cost of the lives of her brood. The skull of one, achieved honorably, makes a Yautja very high in the eyes of the rest of the clan.