You need to add color to the white gesso to make the color of your bark.
Paint with one way strokes with colored gesso. The outer paint starts cracking right away. Don't overlap with the paint strokes, it interferes with the cracking effect.
If you cut an opening in the egg, highlight the edges with black or gold paint. You could even make a thick edge around the opening as if it was a hole in a tree that tried to heal. I would suggest doing that before you make it crackle.