The water/water and water/gas interfaces of the developing embryo are described. During incubation three main compartments are present the subgerminal amniotic and allantoic fluids. The formation composition and regulation of these solutions are considered as examples of the water/water interfaces. Evaporative water loss across the eggshell is considered as an example of a water/gas interface. The effects of this phenomenon are considered as having been oversimplified in the past because they ignore phase changes (condensing of water vapour) and the ability of the embryo to regulate its development according to the supplies available.