Developing eggs can withstand cooler temperatures for short periods. They need to do this as the broody hen/duck will need to get off the eggs occasionally to eat, drink etc. Cooling down like this will not kill the embryo unless it's been really cold and for a long period. I've heard of incubators being accidentally unplugged and undiscovered for up to 24 hours and the eggs hatched. The cool down may delay the embryo's development a bit though, so keep that in mind when hatch day comes. They might hatch a little bit later than expected. Good luck!
