Unfortunately for Jeff, he had misread the Chronivac. He thought it said that he had slightly less than 24 hours before he completely changed, but in fact he had slightly less than 24 minutes.
"Okay, let's see," he thought to himself as he tried to clear the growing fog in his head, "how can I reverse this?" He looked at the Chronivac program on the screen, but try as he might he found that he couldn't remember what any of the buttons or options did. "Okay, it's a complicated program, no need to get worried, I'll just read what it says..." Jeff tried to read the options, but reading was suddenly a lot harder. It was confusing what everything meant, and he had to connect each letter to the next letter to read anything, but his head was getting so foggy and heavy that he couldn't focus for long enough to finish a word. "R-e-v-uh... what do those symbols mean?" He could no longer recognize the letters that were on the... computer? The shiny thing that did... things? What was it? No, no, he had to focus on the letters. "They mean... something... don't they? Sounds. But how can squiggly marks be sounds? How can something be something else?" He tried to think, to figure it out, but his head was too heavy that he just couldn't think through it, it was too confusing. The concepts of writing and symbols eluded him now, just like theoretical quantum physics would have before.
It was beyond his ability to comprehend, so the obvious solution that he came to without even thinking about it was to give up. He didn't know why he was sitting in front of the glowing thing that his owner used, anyway. He got up and climbed onto the bed. He liked the... bed?... soft thing, it was nice that it was soft, and it was warm too when his owner was there. Wait, owner? A flash of clarity came to Jeff. He was someone's pet now, used for mating. No, he had to fight this! He tried to assert himself by saying, "My name is Jeff and I am a human," but it came out as, "Mah namma Jaffan amma hyooma." He couldn't remember how to make his mouth move to make words. Words? What were those? He tried to speak again, but he could no longer grasp the concept of language or remember what it meant, and could only make a bellowing noise.
Throughout all of this, unbeknownst to him, Jeff was changing physically, though slightly, his features becoming more animalistic and primitive. His skull seemed to deflate, fitting his loss in brain function. His brow got heavier and his features got coarser, and all over he grew slightly hairier and more muscular. His posture shifted into a loping slouch. He still looked more like a human than an ape or a caveman, but a human with ape-like features.
Jeff summoned himself again to fight through the cloud in his head to remember his life, but it was slow, and hard, and his memories were fading and came to him only as brief images that he was already having trouble understanding. He was doing things that men did, but he was just a husman. As his old memories faded away, new ones came in, growing up first with his mother and then in a place with lots of other husmen, where they fought and played until his girl came and adopted him. His owner. But even those memories were murky and foggy. It was hard to think of the past, and almost impossible to think of the future. Jeff could only bring his mind to focus on the here and now. Vaguely, with his last scraps of self, he tried to bring himself to feel some emotion about losing himself and being reduced to a mere animal. Despair or sadness or anger or fear. But he could no longer remember or understand what any of those things meant. He didn't have emotions anymore, they were beyond him. He barely even had thoughts. He just had instincts: want to eat, want to drink, want to sleep, want to play, want to mate.
Want to mate. His man wasn't there, but he heard someone coming. Maybe it was her? They could mate. His thought process was much more simple now. He got off the bed and approached the source of the noise, as his penis, which had grown longer and larger along with his testicles, now grew hard at the prospect of mating. He still had the nagging feeling that something was wrong at the back of his head, but he couldn't remember what or why, and it was too hard to focus on or think about, so he didn't. He didn't need to think, that's what men like his owner were for. He was just a husman, and he was content. Contentedness pervaded his mind since he couldn't conceive of anything else.
He found the source of the noise and saw that it came from...