1. The development of eggs of E. esculentus or E. miliaris which have been washed in a current of sea-water until they are becoming unfertilisable is improved by later treatment with egg-water or solutions of thyroxine in sea-water. The secretions and thyroxine improve the development of the eggs to about the same extent. 2. The improvement may be produced by egg-water of the opposite species. 3. These effects are due to action on the egg and not on the sperm. 4. The development of unripe eggs is also improved by treatment with egg-water or thyroxine. 5. None of these effects are due to variations of the hydrogen-ion concentration or osmotic pressure.