Writers are notoriously vague about the number of human sperm produced. It doesn't matter too much - when you are dealing in hundreds of millions of the little fellows the odd few million are not all that important. One writer suggests that the number of human sperm can vary from zero to 700 million or more per millilitre. The zero bit needn't surprise us: after all we know that some men are sterile. The upper extreme, however, is quite remarkable. Considering the complexity of a single sperm the capabilities of the testes in generating such intricate machines by the hundreds of million are little short of miraculous.