Because cells are so small, no one observed them until microscopes were invented in the mid-seventeenth century. Robert Hooke first described cells in 1665, when he used a microscope he had built to examine a thin slice of cork, a nonliving tissue found in the bark of certain trees. Hooke observed a honeycomb of tiny, empty (because the cells were dead) compartments. He called the compartments in the cork cellulae (Latin, “small rooms”), and the term has come down to us as cells. 

The first living cells were observed a few years later by the Dutch naturalist Antonie van Leeuwenhoek, who called the tiny organisms that he observed “animalcules,” meaning little animals. For another century and a half, however, biologists failed to recognize the importance of cells. In 1838, botanist Matthias Schleiden made a careful study of plant tissues and developed the first statement of the cell theory. He stated that all plants “are aggregates of fully individualized, independent, separate beings, namely the cells themselves.” In 1839, Theodor Schwann reported that all animal tissues also consist of individual cells. But already in 1885 the pathologist R. Virkhov supplemented it with the well-known postulate: Omnis cellula e cellula. Which in translation from Latin meant: “Each cell comes from a cell.”

The cell theory, in its modern form, includes the following three principles:

1. All organisms are composed of one or more cells, and the life processes of metabolism and heredity occur within these cells.

2. Cells are the smallest living things, the basic units of organization of all organisms.

3. Cells arise only by division of a previously existing cell.

Already in the middle of the XIX century. the cell theory became generally accepted and gave rise to a new biological cytology.

Cytology is a biological science that studies the features of the structure and functioning of cells.

According to the forms of existence, life on Earth can be divided into two large groups – non-cellular and cellular. The latter, in turn, are divided into prokaryotes, unicellular and multicellular eukaryotes.

Acellular forms (viruses) are a special group of intracellular parasites that behave like living beings in cells and like protein-nucleotide crystals outside cells. Cellular forms are living organisms that are separated from the environment by special membranes. They vary considerably in size and shape. But, in any case, prokaryotic cells are much smaller than eukaryotic ones.

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