McGonigal's Worm

It had happened—no question of it. Now how could it be made to unhappen?
When it happened, it happened unnoticed. Though it affected all chordata on Earth (with a possible exception to be noted in a moment), nobody knew of it, not even the Prince of all chordata, Man himself. How could he have known of it so soon?
Though his lifeline had suddenly been cut, it was a long lifeline and death would still be far off. So it was not suspected for nearly twenty-four hours, nor accepted even as a working theory for nearly three days, and not realized in its full implications for a week.
Now, what had occurred was a sudden and worldwide adynatogenesis of all chordata, not, however, adynatotokos; this distinction for many years offered students of the phenomenon some hope.
And another hope was in the fact that one small but genuine member of chordate was not affected: an enteropneustron, a balanoglossida of the oddest sort, a creature known as McGonigal's Worm. Yet what hope this creature could offer was necessarily a small one.
The catastrophe was first sensed by a hobbyist about a day after it occurred. It was just that certain experiments did not act right and the proper results were not forthcoming. And on the second day (Monday) there were probably a hundred notations of quite unusual and unstatistical behavior, but as yet the pattern was not at all suspected.
On the third day a cranky and suspicious laboratory worker went to a supply house with the angry charge that he had been sold sterile mice. This was something that could not be ignored, and it is what brought the pattern of the whole thing into the open, with corroboration developing with explosive rapidity. Not completely in the open, of course, for fear of panic if it reached the public. But throughout the learned fraternity the news went like a seismic shock.
When it did reach the public a week later, though, it was greeted with hoots of laughter. The people did not believe it.