Animalculae. Animals so small as not to be immediately perceptible to the naked eye. Their origin is unknown, for they rise in substances which did not previously contain them, and their numbers and varieties are infinite. Their existence is generally, perhaps always, preceded by that process of atoms called fermentation. Some have been discovered, not the 10,000th part of an inch long, so that a million of them might be contained in a cubic inch, yet perfect in their parts, and depending on air.