At first I thought it was just embryo movement but I held the egg very, very still and could actually count the heartrate of this particular embryo. Even my husband (who is NOT chick happy, I might add) could see it!! I'm so pumped after a 0% hatch first go round...
Heart beats are fast in a embryo kept at 100F. Over 100 a minute. If you drop the heart rate in an ice bath to 4 beats a minute, you can fully restore it to over 100 a min just fine! Best of luck with your hatch!