I posted this on KS a couple of weeks ago - I had a clutch from a WC girl that looked good but when candled about half of them glowed yellow - I figured infertile / dead - but stuck them in the incubator anyway - a week later they still looked good - I candled them again and found some faint veining inside of them and little blotches of some sort - about 2 weeks in they started to deteriorate and eventually they all died. I'm told that the Barkers book addresses this in their book as a fertile egg that wasn't able to form a proper veining system - or something like that.