Some unfertilized eggs are passed unshelled, as slugs. Some are shelled, and passed. But these aren't unfertilized eggs you're talking about.

Snakes don't pass undeveloped embryos that aren't in some sort of egg. Someone who may be more of an expert than I can correct me if I am wrong, but I'm pretty darned sure they do not reabsorb anything that's been fertilized, either.