Development of the chick embryo: effects of egg mass

We asked to what extent, within a species, differences in egg mass, hence in eggshell surface area and O2 conductance, could affect the development of the avian embryo. Large (L, about 70 g) and small (S, about 55 g) fertile chicken eggs were simultaneously incubated and the embryos studied at day 18, i.e. before the onset of lung ventilation. Embryo mass and O2 consumption (VO2) (measured with a manometric technique) normalized per mass of the freshly laid egg were higher in S than in L, while no differences occurred after normalization by egg surface area (S.A.). Egg water vapour conductance, which is proportional to O2 conductance, was also found to be directly proportional to egg S.A. Hence, the mass and VO2 of the embryo are more closely related to the O2 conductance of the eggshell than to egg mass, giving support to the concept that the avian embryo's VO2 is not an invariable species characteristic, but a variable dependent on O2 availability. Hatching and viability did not differ between L and S, and the specific mass of heart and lung and their cellular (DNA) concentration were also similar between the two groups of embryos. Therefore, differentiation of tissues and organogenesis do not seem to be affected by the differences in total O2 availability determined by the differences in egg mass, while they are probably more important than total tissue mass in setting the time of hatching.