Problem:

Some time ago, I read James Gleick's "Genius: The Life and Science of Richard Feynman", a wonderful biography of Feynman and, by extension, most of modern physics. In this book, the author mentions that Feynman spent a year working in a biology lab and that during that year he made an important discovery.

I remember that when reading the book as an undergraduate biology student, I knew the discovery in question well enough to be immensely impressed that it had been discovered by someone foreign to the field.
Unfortunately, I no longer have the book and cannot remember what that discovery was.

Does anyone know about this?