Nick Bostrom is far too good-natured to be trying, deliberately, to create conniptions among bio-ethicists, but still (p.38) …

… stem cell-derived gametes would allow multiple generations of selection to be compressed into less thasn a huamn maturation period, by enabling iterated embryo selection. This is a process that would consist of the following steps:
1. Genotype and select a number of embryos that are higher in desired genetic characteristics.
2. Extract stem cells from those embryos and convert them to sperm and ova, maturing within six months or less.
3. Cross the new sperm and ova to produce embryos.
4. Repeat until large genetic changes have been accumulated.
In this manner, it would be possible to accomplish ten or more generations of selection in just a few years. …