1. internal pip: piercing by a chick of the inner shell membrane a day or two before the chick begins to hatch out of the shell; gives access to the air cell between the inner and outer shell membranes and prompts lungs and air sacs to begin functioning; 2. external pip: first break by the chick through the hard shell to begin hatching, a tiny crack or hole that gradually grows as the chick pecks with its egg tooth and pushes against the shell with neck, head, and legs