Non-invasive measurement of oxygen partial pressure, lateral diffusion and chorioallantoic blood flow under the avian eggshell

We measured PO2 under the shell of avian eggs indirectly, by sealing 0.05 mL glass tubes to the shell, sealing them with mercury and using an oxygen microelectrode to measure the contained gas that equilibrates with the gas in the shell membranes. This technique requires a smaller area of contact with the shell and a shorter equilibration period than established techniques, and allows measurements at several locations simultaneously and over a long period of time without endangering the embryo. PO2 under the shell of chicken eggs decreased to 14.3 kPa on the day before hatching (day 19). PO2 was unstable during late development and differences up to 3.1 kPa occurred transiently on opposite sides of the equator. By waxing the shell around sampling tubes, we estimated Krogh's coefficient for lateral oxygen diffusion in the shell membranes at 1.1 mmol cm− 1 d− 1 kPa− 1, a value about a third of a previous estimate. Sampling of gas under sufficiently large regions of waxed shell allowed indirect measurements of chorioallantoic venous PO2, without affecting embryonic respiration. Venous PO2 was 3.8 kPa on day 19. Assuming 14.3 kPa represents arterialized blood leaving the chorioallantois, it became possible to calculate the effective chorioallantoic blood flow rate, which was 3.5 mL min− 1 on day 19.