Fertile eggs of White Leghorn chicken (Gallus gallus), about 60 g fresh weight, were purchased from a local supplier. Incubation started at midday (embryonic day E0) at the constant temperature of 38 °C and 60% relative humidity, with automatic egg rotation every few hours. A separate set of eggs was similarly incubated in conditions of hypoxia (15% O2), beginning from E5, until hatching included ( Szdzuy and Mortola, 2007 and Ferner and Mortola, 2009). This level of hypoxia is tolerated by most embryos, although it results in blunted body growth and reduced thermogenesis and ventilatory chemosensitivity in the hatchling ( Mortola, 2009, for review). Hypoxia was obtained by leaking a small stream of N2 into the incubator under the control of a flowmeter. The O2concentration in the incubator was continuously sampled by an O2 analyzer (Foxbox fuel cell gas analyzer, Sable Systems Int., Las Vegas, NV) and displayed on a computer monitor. Temperature and humidity were sampled by a data logger placed inside the incubator.