Osmotic pressures in the henʼs egg

Abstract

In a recent paper Howard (1932) claims to have shown, by three methods, that the "expected" osmotic equilibria exist between the yolk and white of a hen's egg. Johlin (1933) has criticized her technique of cryhydric measurement and re-asserted that the yolk and white of an egg hive different values for depression of freezing point. Although Needham (1931) and Meyerhof (1931) have considered the possibility of the outer layer of yolk having a lower osmotic pressure than the inner, Howard gives no experimental evidence indicating the existence of an osmotic gradient within the yolk. The methods she used being apparently incapable of showing the difference in osmotic pressure between the whole yolk and the whole white of an egg were presumably unable also to detect the osmotic gradient in the yolk. Grollman's (1931) criticisms of the Hill thermo-electric method for the measurement of vapour pressures when employed with viscous solutions were repeated by Howard, with no other evidence than that it gave results which disagreed with her own. In particular, Bateman's low vapour pressure depression found in mixtures of egg yolk and egg white are declared to be incompatible with high vapour pressure depressions for yolk. It is strange that in the differentiation of the properties of egg yolk and white so many authors should have considered the yolk as homogeneous. It is a well-known fact that the formation of an egg yolk occurs by daily deposits in the ovary of the hen. These extend over several days and that the integrity of the daily deposit is maintained more or less for many days is evidenced by observation of the spherical zones in the yolk of a frozen egg that has been sectioned. Also it is easy to withdraw from the centre of the yolk, using a fine pipette, white yolk which is different chemically from the surrounding yellow yolk. Since no membrane is known to separate these two kinds of yolk nor the daily deposit of yolk, the existence of this non-homogeneity within the yolk must be an indication of the slowness of equilibration inside a hen's egg. Hence, when one speaks of the difference in osmotic pressure of average egg white and average egg yolk, no conclusions can be drawn logically regarding the difference in osmotic pressure on opposite sides of the vitelline membrane. As the Hill thermoelectric method of measuring vapour pressure requires but small quantities of solution, it was of interest to use this micro method to study the difference in osmotic pressures of samples of yolk and white obtained on opposite sides of the membrane.