door?" one called pleadingly.

"Explain to the watchguard out there, will you?" Closing the door, he turned on the ventilator and sat down beside Dr. Haslam.

"Why all the melodrama?" Karl asked, baffled. "First you call me by emergency code, then you hole in like a conspirator. I'm beginning to think you're a great loss to Theater. What's happened? Why is it later than I think?"

"Do you take everything as a joke, Karl?"

"Certainly, until I'm forced to do otherwise. What's worrying you?"

"I'm afraid of being arrested for treason. Don't laugh! This morning I received a message, delivered in person by our old schoolmate Lanza, to report to Leader Marley on Wednesday, and Marley hasn't paid any attention to me since he last inspected our lab, years ago. For another thing, Leah Hachovnik is making a nuisance of herself with her curiosity about my affairs. If she weren't so clumsy about her prying, I'd almost believe she was under orders to spy on me."

Karl moved impatiently. "I hope you're not turning psychotic. You have a clean record of continuous production and you've never mixed in politics. You've never expressed what you may really think of our Leader even to me, although we've been friends since we were in Medschool, and I hope you never will. And you're making progress with White Martian. Why, my boy, you're all set! What's treasonable about that?"

Someone knocked at the door. Hastily David uncovered the fragrant beaker and waved it about as he called, "Come in!"

The watchguard looked in for an instant, wrinkled his nose, and quickly shut the door. Laughing, David covered the beaker, and began walking about with long nervous strides, snapping his fingers as he tried to explain.

"I'm in trouble, Karl. I've run into something I don't know how to deal with, and I need help, I need advice, I need cooperation. I've lived alone with this thing for ten long years, hoping month after month that something would turn up so I could evade the issue. But nothing has. And now there's going to be a showdown."

Karl touched his arm sympathetically. "My dear boy—"

"That's it!" shouted David.

"What's what?"

"That's what I'm trying to tell you. Why do you always call me your 'dear boy?' You know I'm a year older than you are."

"It's just habit, I suppose. You look so young—your hair is black, while mine is nearly white. You're full of vigor, while I begin to creak with middle age. I didn't realize that I irritated you with my little phrase. I should think you'd be pleased that you have somehow managed to sip at the fountain of youth."

David sank down on a stool. "I'm not pleased. I'm terrified."

"What do you mean?"

"I mean that's exactly what's happened. I have sipped at the fountain of youth. I've discovered how to keep people from growing old. I myself have not aged a bit in the last ten years."

There was a long silence. Karl sat unmoving, his face like stone.

"I don't believe you," he said at last.

"It's no longer a question of belief. In a few days everybody will know, the proof will stare you in the face. And what will happen then?"

"Evidence?" Karl asked. "I can't accept a statement as a fact."

"Would you like to see my mice? Come with me."

David Wong hurried into the small animal room and paused before a stack of wire cages in which furry creatures darted and squeaked.

"You remember when we were working on Blue Martian, those peculiar mutants we found in our mice, and how I used six of them in trying to make antibodies to the virus?"

"I remember," said Karl. "They were spotted with tufts of white hair on the right forelegs."

David took down a cage, thrust in his hand, and brought out two of the tiny black mice which crawled over his trembling hand. Their right forelegs bore tufts of long white hair.

"These," he said, "are the same mice."

"Their descendants, you mean. Mice don't live that long."

"These mice do. And they'll go on living. For years I've lived in fear that someone would notice and suspect the truth. Just as for years, every time someone has laughed and told me I never seemed to age a day, I've been terrified that he might guess the truth. I'm not aging."

Karl looked dazed. "Well, my boy, you've got a bear by the tail. How did you find the elixir or whatever it is?"

"You remember the early work with radioactive tracers, a couple of hundred years ago, that proved that all our body cells are in a continuous state of flux? There's a dynamic equilibrium between the disintegration and the resynthesis of the essential factors such as proteins, fats and amino groups, but the cell directs all the incoming material into the right chemical structures, under the influence of some organizing power which resides in the cell.

"Foreign influences like viruses may disrupt this order and cause cancer. The cells are continually in a state of change, but always replace their characteristic molecules, and it is only as they grow older that they gradually become 'worn out.' Then the body grows old, becomes less resistant to infection, and eventually succumbs to one disease or another. And you know, of course, that viruses also have this self-duplicating ability.

"I reasoned that at birth a man had a definite, finite amount of this essential self-duplicating entity—SDE—in his body cells, a kind of directing factor which reproduces itself, but more slowly than do the body cells. In that case, with the normal 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