Water balance in the eggs of the Atlantic salmon Salmo salar.

The osmotic and ionic properties of the eggs of trout and salmon have attracted the interest of physiologists for many years. The eggs are convenient biological material and immense numbers of large, almost identical eggs can readily be obtained. The very slow development of the eggs raises in an acute form the ionic and osmotic problems common to the eggs of all freshwater animals. When shed into isotonic saline, eggs of salmon and trout maintain" an almost constant weight and a relatively high permeability to water. When shed into fresh water the eggs become almost impermeable to water but increase in weight by about i6%", during the first 2 hr., as the result of the formation of a perivitelline fluid beneath the chorion (Bogucki, 1930). Eggs in this impermeable state are described as 'waterhardened'. The decline in permeability to water is due to a change in the properties of the vitelline membrane (Krogh & Ussing, 1937), the outer chorion remaining permeable to water even in the water-hardened condition (Gray, 1932). Krogh & Ussing (1937), as the result of experiments with deuterium oxide, decided that the permeability of the vitelline membrane became immeasurably low. More recently, Kalman (1959), using tritiated water, showed that there was a small exchange of water between the yolk and the surrounding media even in the water-hardened eggs of the trout, Salmo gairdneri, but no estimate of the permeability was made. The formation of the perivitelline fluid is inhibited in a saline solution but the perivitelline fluid is formed when the eggs are shed into isotonic urea or glucose solutions (Bogucki, 1930). In this paper we describe the uptake and exchange of water in the eggs of Salmo salar when shed into a variety of media.