Chicks zip the eggs just fine, but can't tare through the egg lining. I left them in the egg for 15 hours after they broke the shell all the way around and then I helped them out. The chicks are doing great now. I had the humidity 45% to 50% up until day 18 and raised it to 65%+ the last few days. The eggs are Black Copper Marans. It looks like the lining is real tough.