The hardening of the chorion of salmon egg has been studied.
1) The degree of re-absorption of water depends upon the duration of initial absorption in tap water, and in re-distilled water.
2) The degree of later absorption of water differs according to the duration of interruption of water absorption when liquid paraffine is employed.
3) The degree of water absorption of the eggs in isotonic b.s.s. which have been immersed in 10% solution of gum arabic, differs according to the duration of immersion.
4) Hardening is an irreversible change.
5) From these facts, it may be concluded that the hardening of the chorion is caused not only by the existence of bivalent cations in the surrounding medium, but also by some other factor, which has not been clarified. It is probable that the tonicity of the medium plays a rôle in the hardening reaction.