Not that much. The first edition of high yield embryo is probably the best for USMLE purposes because they added a lot of extraneous info in their subsequent versions. BRS was the easiest to read, but I recall that BRS' info conflicted with a lot of the stuff that we were being taught in lecture during my first year, so I don't know which version was "right". I suspect that it was neither since so little is known about human embryo. I'd recc sticking with first aid and step up for embryo purposes, maybe read through high yield once just to remind yoruself. Don't waste too much time on it.