1. Socium silicate added to the culture medium in which the rotifer Brachionus pala was living caused nearly all and in many cases caused all of the descendants to form two additional posterior lateral spines and also to increase the size of the other spines thus forming Brachionus amphiceros. 2. The parthenogenetic eggs from females reared in sodium silicate solutions developed into young females of the Brachionusamphiceros type possessing these two lateral spines while the fertilized eggs from females reared in sodium silicate solutions developed into young females of the Brachionus pala type lacking these two lateral spines. 3. The variety or species of rotifer, Brachionus amphiceros, does not exist as a true variety or species, but is only one of the polymorphic stages of Brachionus pala. 4. Perhaps the polymorphic forms of this rotifer as it occurs in pools and ponds during the year are produced by the varying amounts of soluble siliceous substances in the water. 5. The females of Brachionus amphiceros possessing the two lateral posterior spines readily produced female parthenogenetic eggs, male parthenogenetic eggs, or fertilized eggs according to the supply of the green food Chlamydomonas.