Sorry Mob, but how would the experiences of the Marine affect the genetic material in his Progenoids? That is disproved Lamarkian evolution. Essentially, the genetic material found in a Progenoid is static- this is why there is a 5% tithe of a Chapter's Geneseed by the AdMech, to make sure that this genetic information isn't changing; if it is changing, then they Tech Priests will have to step in to see why and maybe do something to stop it.
As others have noted, the Geneseed is almost exactly like its name. Inside the Progenoids are small collections of cells representing each of the implants that make a Marine, a Marine. This will included its own, Progenoid-making, information.
When these cells are taken out of the Progenoid they can be cultured and grown into the new organs which will, in turn, be implanted into a new Scout/Neophyte.
In one of the HH books, Astellan, a Terran DA, talks about having his Progenoids removed years ago because they had 'matured', and the way he talks about it suggests that this isn't unheard of and is, in fact, standard procedure. I recall him, or another book, saying that it takes the Progenoids about 20 years to mature. Now, if this process of taking mature Progenoids from Astartes has continued into the 'present' 40k day, then it would really help to increase the rate at which Astartes can be made, as you can be getting on with making a new one whilst the Progenoid donor is still alive, rather than waiting for him to die. However, such a thing is by no means guaranteed to still happen, what with the debasement of the implantation procedure into meaningless ritual and dogma.
Hope that at least adds to the conversation!