By Justine -
At 48 hours into incubation, I candled my eggs to see if I could see veining for the first time... just out of curiosity, as they are my much anticipated Barred Plymouth Rocks.
Unfortunately this egg was cracked in two places and leaking Albumen (egg white) - so I had to toss it. Before doing so, I thought it would be interesting to see what was going on at 48 hours.
If you look very closely, you can just make out the heart - still beating.
The heart continued to beat for roughly 5 minutes. I took this video after it had already started to slow.