by Welkin John­son — One hun­dred years ago this month, Pey­ton Rous pub­lished the first in a se­ries of pa­pers de­scrib­ing ex­per­i­ments that be­gan with work on a sar­coma from a sin­gle Ply­mouth Rock hen. In that ini­tial pa­per, Rous showed that bits of the tu­mor could es­tab­lish new tu­mors when...