Pulmonary ventilation in a population of hatching chick embryos.

Abstract

Synopsis Two experiments were made to study pulmonary ventilation in the hatching chick embryo with particular reference to the part played by the air cell. The hatchability of embryos without access to the air cell was the same as for normally positioned embryos. The lungs of about 35 per cent of the normally hatching embryos were inflated in the air cell, but 65 per cent were inflated after breathing atmospheric air. The shell was cracked but not pipped before the outer membrane was penetrated. Comparisons between different groups of normally and abnormally positioned unhatched embryos revealed that the air cell had an insignificant respiratory value, but that the mechanical advantages of the large end seemed apparent. The time relationships between lung inflation, inner membrane perforation, pipping, yolk sac retraction, outer membrane perforation, lung discoloration and hatchability were studied. Equations were derived for the regression on time of each of the seven variables studied. Statistically insignificant differences in hatchability were observed between the 2 groups of embryos which hatched from the large or small end of the egg. It is suggested that lung inflation occurs when a certain threshold in the respiratory movements of thoracic muscles is reached. The threshold is attained by the rising anoxia, and probably other stimuli. The chick embryo's lungs, which lack elastic recoil, respond to the threshold and start utilising air. It is doubtful whether the embryo needs the very high partial pressure of carbon dioxide in the air cell. It is feasible to believe that CO2 is an inevitable metabolic by‐product, and that it is stored in the air cell. The air cell also serves as a resting place for the chick while retracting the yolk sac. Through evolution, however, the chick embryo may have built up a high physiological tolerance to the CO2 which it encounters upon entering the air cell.