Liane B. Russell Biology Division, Oak Ridge National Laboratory Oak Ridge, TN 37830 Composite individuals have always excited mankind's imagina- tion. Even the earliest recorded mythologies are full of fanciful examples of creatures such as centaurs, mermaids, androgynes, and winged horses. While real, naturally occurring, mosaics might not appear as spectacular to the popular mind, their study -- particu- larly in Drosophila and plants -- has made important contributions to genetics, starting relatively early in this century. In mammals, too, examples of mosaics resulting from early or late...