1. The pH remains steady, 7.57. 2. The pCO2 increases gradually. We believe this is fundamental for hatching. 3. Bicarbonate and base excess increase from acidosis to alkalosis due to absorption of the egg shell. 4. The pO2 increases in stage 41 and 42 due to pulmonary respiration. 5. The pO2 drop prior to hatching due to involution of the chorioalantoid membrane and increasing requirements.