I was candling the egg today and accidently made a crack in the end of the egg.
The shells are so thin and the eggs so small that I was concetrating on not dropping it that I pushed the candlee too hard against it.
Fortunately (if there is a fortunately) it was on the end with the air sac, so I sealed the hole with some uncoloured nail varnish and hopefully it will stop any bacteria getting in.
I've read about people using nail varnish, melted candle wax and ever sellotape to fill in cracks on eggs, but because this is such a small egg I'm not sure if it'll work.
It was fertile because I could see tiny little veins growing on the inside.
Good luck chick!