multiplication of the cells,
the amount of SDE per cell would slowly but surely grow smaller with the
years. Eventually the time would come when the percentage would be below
the critical level--the cells would be less resistant, would function
with less efficiency, and the man would 'grow old.'"

Karl nodded soberly. "Reasonable hypothesis."

"But one day, by pure chance, I isolated a component which I recognized
as being the factor essential to the normal functioning of body cells.
It hit me like a toothache. I found that I could synthesize the SDE in
the lab, and the only problem then was to get it into a man's cells. If
I could do that, keep the SDE level up to that of youth, a man would
stop aging! Since viruses penetrate our cells when they infect us, it
was no trick at all to effect a chemical coupling of the SDE to the
virus. I used Martian Blue, since it was handy, and its effects are
usually brief.

"Presto! Old age is held at bay for another twenty or thirty years--I
really don't know how long. These mice were my first experiment, and as
you see, they're still alive. Next, I tried it on myself."

David put the mice back in their cage, locked it, and returned to the
lab.

"Tomorrow, the whole thing is bound to come out because Tanya Hachovnik
is coming back. You know her sister Leah--gray, dried-up, soured on
life. Well, I've had ways of checking, and when Tanya Hachovnik walks
into the Institute, everyone will see her as the same luscious redhead
of twenty-five we knew ten years ago. I realize that what I did was a
criminal act. I didn't think the thing through or I wouldn't have been
such a fool. But when I made those final experiments, I used the
Hachovnik twins for a controlled pair."

"You must have been crazy!"

"Perhaps I was. I'd tried it on myself, of course, with no bad effects
except a few days' fever, but I realized that without a control I never
could be sure the SDE was actually working. It might be just that my
particular genetic constitution caused me to age more slowly than the
average. So I chose the twins. To Leah I gave the attenuated Martian
Blue, but to Tanya I gave the simple Blue coupled with SDE. The
experiment worked. Identical twins--one grows old like other people; the
other remains young. I know now, Karl, how to prolong youth
indefinitely. But what in the name of Leader Marley shall I do with my
knowledge?"

Karl Haslam absently twisted his white hair and spoke slowly, a