What does zip mean??

[steffpeck]

I have seen "my eggs have zipped" on several threads, can someone tell me exactly what that means? Thanks!

 

[Tuffoldhen]

The pip is a hole the chick is making to learn to breathe and get out of the egg...the zip is the making of a circle like around the shell to break free...it can take the chick up to 24 hours from his pip to zipping out...he is learning to breathe, and he rests in between times...