The cell theory says that:

1. All organisms are composed of one or more cells.
2. All cells come from preexisting cells.
3. All vital functions of an organism occur within cells.
4. Cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.

Microscopes continued to be improved upon by scientists that studied cells. As a result, the scientists could see closer and begin to understand the internal structures of an individual cell. The internal structures of a cell are called organelles, which means "little organs." Just as our bodies are made up of organs, the "bodies" of cells are made up of organelles.

There are two basic types of cells:

• prokaryote: do not have a nucleus or membranes surrounding its organelles. Example: bacteria.
• eukaryotes: do have a nucleus and organelles with membranes surrounding them. Example: plants, animals.

Cell theory was developed in the 19th century when scientists were experimenting with their latest technology, the compound microscope.

See also: germ theory