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Osmoregulation by Avian Embryos: The Allantois Functions like a Toad's Bladder

Avian eggs lose water during incubation. Within a species, the rate of water loss can vary as a result of variations in nest humidity and because egg shells differ in their resistance to the diffusion of water vapor. While a certain amount of water loss is necessary, the chance of an egg hatching is decreased if it loses too much or too little water. There are several extraembryonic water compartments (yolk, amniotic fluid, allantoic fluid), and the water content of the embryo might be regulated by varying the water content of another compartment and, thus, protecting the embryo from some of the effects of variations in water loss. This study was designed to determine if variations in water loss affect all compartments equally or if the water content of the embryo is actively regulated. Whole-egg water loss of chicken eggs was varied by incubating them at different humidities. After 17 days of incubation, I measured the volume of water in the four compartments. As desiccation is increased, about six times as much water is lost from the allantoic fluid as from the embryo, and none is lost from the yolk or amniotic fluid. To better understand the mechanism responsible for this differential water loss, I measured the sodium, potassium, and osmotic concentrations of the plasma, amniotic, and allantoic fluids. Analysis of these data shows that removal of water from the allantoic fluid of eggs with high weight losses is only possible because solutes have been removed from the allantoic fluid. It appears that this is partially achieved by the transfer of sodium to the amniotic fluid against a concentration gradient. There are a number of interesting structural and functional similarities between the epithelia of the allantoic membrane and that of the toad bladder. The ability of the avian embryo to differentially partition the water loss results in a more stable internal environment for embryonic development.