1. The unripe sperm of the starfish is activated by treatment with thyroxine in the same manner as that of Echinus esculentus is activated.
2. The fertilisability of slightly unripe starfish eggs is improved by treatment with thyroxine. Ripe eggs are unaffected.
3. In slightly unripe eggs, treatment with thyroxine causes a temporary increase in the oxygen consumption of the eggs. The oxygen consumption of ripe eggs is unaffected.
4. Thyroxine causes, or more probably, accelerates, an oxidative reaction in the unripe eggs, which is, at least in part, responsible for the improvement in the fertilisability of the eggs.
5. Iodine compounds play a part in the fertilisation of the starfish egg, which is in many respects similar to the part they play in the fertilisation of the sea-urchin egg.