CELL. The cell is the unit of life. The name was given by Robert Hooke, an English architect, who discovered the cellular struc ture of plants while examining charcoal and cork. In such objects all living contents have disappeared; so the name, cell, was applied to the honeycomb-like chambers. During the last half of the 17th century, Robert Hooke and Nehemiah Grew published extensive re searches upon the cellular structure of plants; while the Italian physician, Malpighi, and Leeuwenhoek, a Dutchman, published sim ilar investigations upon animals. The entire 18th century, with Linnaeus as the dominant figure, was so devoted to the classification of plants that scarcely any microscopic work was done. The prevailingly small size of cells, to gether with the imperfections of the early mi croscopes, made investigations difficult. Most cells are too small to be seen by the naked eye. Some cells are not more than one micron (one thousandth of a millimeter, or one-twenty five-thousandth of an inch) in diameter. The cells which the earlier observers described were generally 10 microns or more in diam eter. In plants, a cell 100 microns in diameter is very large. The internodal cells of Chara, the Stonewort which grows in ponds, some times reach a length of three or four inches. The eggs of birds, before fertilization, are unicellular. They are the largest cells known. The egg, when it is laid, contains an embryo and so is already a multicellular structure.
The beginning of the 19th century saw a vigorous resumption of microscopic investiga tion, coupled with great improvements in the microscope. In 1838 Schleiden announced his theory that the entire plant consists of cells. This theory, now an undisputed fact, is called the Cell Theory. In the next year, Schwann declared that the entire body of even the most complex animal was built up of cells. While the Cell Theory was soon recognized as a fact, the development of tissues and organs from cells, and the growth, differentiation and mul tiplication of cells furnished subjects for re search during the rest of the century. Many fundamental problems along these lines still remain unsolved, but the improved micro scopes, together with improved methods, gave better views of the cell contents and it was soon recognized that the contents are even more important than cell wall.
The principal contents are the protoplasm and the nucleus. The nucleus was discovered and named by Robert Brown in 1831; the term, protoplasm, was first used. by Purkinjj in 1840 to designate the entire body of young em bryos. In 1846, von Mohl applied the term to the living contents of the cell, so that it is synonymous' with the term, protoplast, as we use it to-day. Most writers now restrict the term still further, applying it to the living contents of the cell, exclusive of the nucleus. In this sense, it is synonymous with cytoplasm. AU cells of both animals and plants, with the barely possible exception of bacteria, have both nucleus and cytoplasm; but many cells, both in animals and plants, lack the cell wall.
Besides the nucleus and cytoplasm there are other cell contents which may or may not be present. In the early nineties many believed that the centrosome was a constant and essen tial organ of the ceU. This organ attains its highest development in animals, particularly during the formation of the polar bodies; but it must now be admitted that in many animal cells no centrosomes can be demonstrated. In plants, there are genuine centrosomes in many of the algae and fungi; some investigators still claim to have demonstrated centrosomes in the liverworts, mosses and ferns; but all now admit that there are no centrosomes in the flowering plants. However, in the liver worts, mosses, ferns, Cycads and Ginkgo, dur ing the formation of sperms, a centrosome like body, called the blepharoplast, is very con spicuous. It is probably a genuine centro some. In most Gymnosperms even this ble pharoplast has disappeared and there is no trace of it in the Angiosperms. Plant cells may contain plastids, starch, chlorophyll and other pigments, oil, crystals, mucilage, resin, etc. These, however, are of only secondary importance, as they are formed under the in fluence of the living protoplasm. Animal cells contain an equally great variety of substances.