Abstract
Testicular teratomas in strain 129 mice originate during the twelfth day of gestation from primordial germ cells. Transplantable teratomas established from grafts of spontaneous teratomas and early embryos form embryoid bodies which are useful to mammalian embryologists. Teratomas may be experimentally produced by grafting genital ridges, and they are useful to study early stages of tumorigenesis.