A neuron is also a single cell, and may stretch up to two feet in length. An ostrich egg, which may weigh up to two pounds, is actually a single cell.

Aside from the occasional ostrich egg or nerve cell, most cells are very small and can't be seen without magnification. As a result, our knowledge of cells has grown as technology has allowed us to see them. The art of glassmaking in the 1600s led to the creation of the first simple microscope lenses, making it possible to see cells up close and personal for the first time.

Robert Hooke, an English scientist, first described cells in 1665. When he looked at a very thin slice of cork through his microscope, Hooke noticed that the cork was porous and comprised of many tiny square boxes that reminded him of the small rooms in a monastery. He called them cells, and the name has stuck, even to today.

In 1675, a Dutch lens maker named Antonie van Leeuenhoek described the first living cells. In the 150 years after his work, microscope lenses improved and scientists were able to observe and understand more parts of the cell. In the mid-19th century, three different scientists working separately each published important conclusions about cells.

In 1838, Dutch botanist Matthias Schleiden concluded that all plants are composed of cells. One year later, a German zoologist by the name of Theodor Schwaan postulated that animals are also composed of cells. And in 1855, Rudolph Virchow, a German doctor, asserted that all cells must come from other cells by the process of cell division. The work of these three scientists was combined into what is now known as the CELL THEORY.

All life on Earth can be divided into two categories: eukaryotes and prokaryotes. The cells of a EUKARYOTE are complex and contain a nucleus and other membrane-bound structures. Eukaryotes include animals, plants, protists, and fungi. With the exception of protists and the yeasts (which are single-celled fungi), all eukaryotes are multicellular.

A eukaryotic animal cell is a complex structure that may be found in a wide variety of shapes and sizes.

PROKARYOTES, or bacteria, are much simpler in structure. These cells have no internal membranes. Don't shed any tears on their behalf, though. These simple prokaryotes colonized Earth two billion years before eukaryotes ever showed up on the scene.

Eukaryotic cells, the focus of this discussion, come in a wide variety of shapes and sizes because they all perform different functions. For instance, an ostrich egg yolk weighs about one pound and is several inches in diameter. Why is it so huge? Its size reflects its need to contain enough nutrients to feed the developing ostrich chick.

A side-by-side view of an animal and plant cell. Plant cells contain special organelles called chloroplasts and an outer covering called a cell wall. Animal cells lack both of these structures.

Most plant and animal cells range in size between 10 and 50 micrometers. The longest cells are nerve cells that can sometimes extend several feet. Their function is to carry electrical impulses through the body quickly. Compare these to prokaryotic bacteria, which, at .2 micrometers in length, are the smallest cells.

Some cells can live independently of other cells. These are unicellular organisms and can grow, feed, reproduce, and respire completely on their own. Other cells are highly specialized and can function only in association with many others as part of a multicellular organism.

So get ready to take a tour though this fascinating and sometimes bizarre cellular world that can be seen only with a microscope. The first stop is a factory.