leading from: The Space Jockey's Story
a) The Laser Placenta: In the silo, below the jockey's chamber, there are thousands of spores that must be as old as the Jockey's remains. They are covered over by a film of blue light that is a laserbeam spread like a thin sheet, and there is a sound as the astronaut penetrates the laser beam's field. It represented the placenta of the eggs
- Ridley Scott: The sheet as I call it of the laser beam, this is a laser beam spread thin like a thin sheet, but it worked great here and I never thought it would photograph because it's pretty low key, but you know, with the wizardry of, of Derek, we got it. So this is all just handheld lay the sound on as you go through the laser beam, you can here, there's a sound to the laser beam, you can here it now, like a seal. I always thought of the laser beam as the placenta wall for the, erm, eggs.(Alien 30th Anniversary Edition DVD)
- Ridley Scott:"Loose on the ship, this new alien, begins to lay eggs in the bowels of the ship. It lives to propagate and must find food for it's offspring - in this case, the crew members of the Nostromo upon whom the young aliens can feed in their eggs until a new host comes along prodding the eggs. The the cycle begins all over again" (S&S v48.n.1 Winter 78/79, p26)
When Giger came to design the cocooned victims, he imagined it to be the way a spider wraps its victim and sticks it to the wall
- H R Giger: "Like the way a spider wraps its victim and sticks it to the wall" (Giger's Alien Diaries p545)
So it does seem to me that explanation behind the idea of the human to spore "transformation", even without Ridley Scott's explanation, the victim inside the cocoon was being eaten away and indeed the silk cocoon turns into the spore shell. And so following this, Scott actually went with the idea of showing human bodies being eaten away alive by the cocoon material putting Dallas in a state of agony, as the spore shell grew around the humans' remains.
- Dan O'Bannon "Ripley finds Captain Dallas spun into a web in the hold of the Earthship. Brett and Parker have also been spun into cocoons which are in the process of metamorphosing into the very same type of egg found in the pyramid"(Starburst 15, p41)
James Cameron wanted an egg layer because he was not convinced by the idea that human victims could genetically transform into spores, where he actually got the idea that victims were doing this, I have not found. He saw no evidence in this regarding Kane's corpse for instance. Cameron must have jumped on this just as a vague assumption that was too hard for him to find further information about, or maybe the producers mislead him not feeling bothered to address the issue precisely.