I got thinking about the progenoid glands and how they grow and are able to produce new gene-seed. As an overview (or perhaps even an introduction to the topic!), each and every space marine is implanted with a number or organs during their teenage years. These grow the person in to a full Astartes and give them abilities beyond ordinary mortals. Here's the catch though - to grow a full set of organs for this process requires a gene-seed. And the gene-seed is contained only within a mature progenoid gland.
Every space marine is implanted with two of these: one that matures in a timespan of 5 years, and a second that is fully mature only after 10 years. These can be harvested after this time period and returned to the chapter (or legion) to grow new sets of organ, although often the 10 year maturation glad is only removed when the marine dies.
To form a new chapter, Terra grows a suite of gene-seed from a pool of vat-slaves. Each is implanted with the progenoid glands as well as the organs, and they're left to mature. Rinse and repeat lots of times, and you'll have enough for a full and new space marine chapter. The only problem is the time line involved.
Let's start with a single (carefully selected) gene-seed. After 5 years, one of the progenoids is ready to harvest. Hence we get our gene-seed back. After 10 years, the second one is ready, and we have double the number we started with.
But suppose we implant the seed back again as soon as it is mature. After 5 years we have our original "slave" in a vat who has produced the 5 year mature seed which is then implanted in another host. After 10 years, the original slave has produced the 10 year seed, and the second slave has produced the 5 year seed. We now have two seeds and a third in progress that'll be ready in another 5 years.
After 15 years, those 2 seeds have yielded 2 lots of 5 year maturation seeds, and the second slave has produced the 10 year seed. That's 3 total, and we have another 2 on their way.
In 20 years, we have 3 lots of 5 year maturation seeds returned, 2 lots of 10 year seed returned for a total of 5. And 3 lots of 10 year seeds growing well.
It continues in this fashion:
25 years: 5 x 5yr + 3 x 10yr (total = 8), with 5 x 10yr gestating
30 years: 8 x 5yr + 5 x 10yr (total = 13), with 8 x 10yr gestating
35 years: 13 x 5yr + 8 x 10yr (total = 21), with 13 x 10yr gestating
40 years: 21 x 5yr + 13 x 10yr (total = 34), with 21 x 10yr gestating
45 years: 34 x 5yr + 21 x 10yr (total = 55), with 34 x 10yr gestating
50 years: 55 x 5yr + 34 x 10yr (total = 89), with 55 x 10yr gestating
55 years: 89 x 5yr + 55 x 10yr (total = 144), with 89 x 10yr gestating
60 years: 144 x 5yr + 89 x 10yr (total = 233), with 144 x 10yr gestating
65 years: 233 x 5yr + 144 x 10yr (total = 377), with 233 x 10yr gestating
70 years: 377 x 5yr + 233 x 10yr (total = 610), with 377 x 10yr gestating
75 years: 610 x 5yr + 377 x 10yr (total = 987), with 610 x 10yr gestating
80 years: 987 x 5yr + 610 x 10yr (total = 1597), with 987 x 10yr gestating
Therefore after 80 years we have more than enough gene-seed to create a new chapter from scratch, with room for failure and tithing to Terran vaults / purity checks.
For something like a full legion (100,000 gene-seed), this would take an additional 45 years. Hence the Emperor must have had a strong lag time up his sleeve when he created the Astartes … or had more than one gene-seed per legion to start out with.
It also illustrates how easy it would be to suffer a huge loss (cf.: Emperor's Children early on) from a small error. And why the Raven Guard sought an accelerated process to get numbers back up after Isstvan V.
In the Warhammer Compendium, it states that it takes about 55 years time to get enough seed to produce a new chapter's worth of organs with constant reproduction. My value above is longer than this, but not outrageously so. Perhaps I've made an error? (Or they have?) Comments welcome!