Living organisms consist of millions, billions or trillions of cells. Human bodies have upward of 37 trillion, most of which can only be seen under a microscope. Other organisms, however, have only one cell in their entire body, and humans can see some of these single-cell organisms with the naked eye. Human egg cells, unusually large bacteria, some amoebas and squid nerve cells make up this list. Some people may think the eggs of egg-laying species are single cells, but that is a false classification. In general, the smallest objects the human eye can see without the aid of technology are no smaller than 0.1 millimeters.
