In the derelict or the Nostromo? If it's the derelict then I guess we'll find out in Prometheus since there were thousands but if it's in the Nostromo, I would say, according to the cut footage, the Alien "laid" the first egg, cocooning up Dallas and Brett, making them into more eggs. Hard to say. Or maybe the first egg was laid inside Kane to produce the Alien.
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According to a spoiler plot that was posted during the trailer release, the eggs on the derelict are a byproduct of vicious skeletal creatures that have mated with the bio-brain. The creatures are mutations of half of the crew that became infected from the weaponized bio-former.
I've always liked the ideal that the eggs were actually the Space Jockey crew... after being turned into them by the Alien. I figure the original creature that infected the S.J in the chair was the actual cargo that got loose.
