1. The existence of a real osmotic difference between white and yolk of the hen's egg has been confirmed.
2. Hill's vapour-pressure thermopiles are shown to be reliable when used with viscous protein solutions such as egg-yolk.
3. The vapour-pressure changes which occur on mixing white and yolk indicate a removal of osmotically active substances. They do not agree with the freezing-point determinations of Straub.
4. The bearing of this result on the osmotic changes occurring in the fertilised and unfertilised egg is discussed.
5. The effects of diluting white and yolk with water and with various salt solution is studied from the point of view of 3 (above) and in relation to the problem of bound water. It is concluded that the amount of bound water in both white and yolk is small. The effect of solid sodium chloride on the vapour pressure of these substances confirms this conclusion.
6. Urea appears to dissolve in egg-white with normal depression of vapour pressure; urea and sodium lactate are largely removed from solution when added to yolk. Glucose is not so removed.
Work done during tenure of a personal grant from the Medical Research Council.