1. In the course of embryonic development the pH values of albumen and yolk go through definite changes, presumably affected by natural metabolic processes occuring within an egg. 2. It was found that the changes in pH either of albumen or of yolk were similar under natural and artificial (laboratory) methods of incubation. 3. The pH of egg albumen rapidly changed towards alkalinity and back during the first week of incubation, reaching the highest point of alkalinity at about 48 hours. For the rest of the incubation period it gradually moved towards neutrality. 4. The pH of egg yolk gradually changed throughout the incubation period from acid to alkaline, with a sudden temporary drop at the sixteenth day.