Spore to Facehugger

leading from

Dan O'Bannon's Original Alien

First the egg (a spore really) which is tended by the third stage adults and housed in a lower chamber of the breeding temple. When ready to hatch, the spore pod is placed in the middle of the sacrificial stone with blood drains in it.

The spores are leathery, egg-shaped objects about one metre tall, which contain the larva of the alien
The alien race had two sexes of its own but they needed a third host animal to reproduce, a lower animal, equivalent of an Alien cow and it is led on to the stone. The other side to this is that the alien appears to reproduce asexually using a hosts for a spores and a chestbursters and in that case we are yet to understand what form the two different sexes that Dan O'Bannon referred to, take in this life cycle.

The small larval alien within the spore pod known as "the face-hugger", that amounts to being an ambulatory penis , is possibly an octopoidal creature which waited for a creature to approach and when someone touched the spore pod, or Sensing the warmth the "face-hugger" springs out, the lid flies off, attaches itself to the animal, deposits eggs the nearest available orifice or as another point of view puts it, deposits a fetus into the stomach..

Sketch by O'Bannon

source quotes

1. The spores are leathery, egg-shaped objects about one metre tall, which contain the larva of the alien (Dan O'Bannon's letter to Giger in Giger's Alien, p10, 363E)
2. Dan O'Bannon:See, these alien beings had two sexes of their own, but they needed a third host animal to reproduce. So they'd bring in an animal, put it up on the plinth with a spore, and whammo!  Then they'd lead the inseminated animal off to an enclosure somewhere to await the birth.  (Don Shay talks with O'Bannon. Cinefex 1.p48). 
3. The small larval alien within the spore pod known as "the face-hugger"(Dan O'Bannon's letter to Giger in Giger's Alien, p10, 363E) 
4. Dan O'Bannon: They have a spore that contains what amounts to an ambulatory penis, and they require a host to reproduce. And when a host approaches the spore, this things springs out and attaches itself to the host and deposits eggs in the nearest available orifice, and then it dies and falls off. (Book of Alien, Scanlon and Gross ), 
5. Dan O'Bannon: This is small, possibly an octopoidal creature which waits for a creature to approach. When someone touches the spore pod, the lid flies off, and the small alien (First Phase), leaps out and attaches itself to the face of the victim (Dan O'Bannon's letter to Giger in Giger's Alien, p10, 363E), 

1. Ron Cobb: First the egg , which is tended by third-stage adults and housed in a lower chamber of the breeding temple. When ready to hatch, , the egg is placed in the middle of the sacrificial stone, and a lower animal, equivalent of an Alien cow , is then led on to the stone. Sensing the warmth the "face-hugger" springs out, attaches itself to the animal, and deposits a fetus into the stomach. The facehugger soon drops off and the fetus develops inside, eventually chewing its way out and killing the host. (Ron Cobb.Alien Portfolio , written and compiled by Jeffrey Walker)