1. Plerocercoid larva of the cestode Schistocephalus solidus were cultured in vitro in horse serum + 1% glucose at the following temperatures: 35, 34, 33 and 30° C.; the temperature at which maturation normally takes place being 40° C.
2. Eggs capable of embryonation were never produced by larvae cultured at a temperature less than 35° C.
3. Below this temperature, spermatogenesis became markedly abnormal, resulting in the production of polyploid cells and degenerating spermatid morula.
4. No abnormalities in the vitellaria or ovaries were observed at these subnormal temperatures.