Not sure if it matters at this point or not but if the egg is white and plump, it's probably fertile. Slugs look yellow, small, and hard. They can lay days apart if they are trying to find a place to lay (I've had it happen with my big girl when she decided to double clutch without warning) but don't have a suitable spot. Eventually they tend to pass all the eggs even if a place isn't provided but can also become egg bound. With corns, it's not always easy to feel the eggs either. Remember they can lay over 40 at a shot.