The parts played by Cells in the functions of living beings are so many and so important that it is necessary at the very outset to consider the properties of the Fig. i. individual elements somewhat in detail.

The demonstration of the cellular structure of plants was first made in 1832 by a distinguished German botanist named Schlieden, who considered the cells to be characteristic of plant tissue. A few years later Schwann showed that the animal tissues, though not so obviously, were also made up of cells, and that they owed their beginning and development to the activity of cell elements. Thus originated the "cellular theory," which, with some modifications, is now the basis of all physiological inquiry.

The first idea which was conveyed by the term cell varied much from that which we now accept as a proper definition of such an organic unit.

Fully developed vegetable cells being the first discovered were taken as the type of all. The main characteristics of these may be briefly summed up. First, a membranous sac called the cell wall, generally very well defined, and, secondly, within the cell wall various cell contents. Among the more conspicuous of the latter may be mentioned (i) a soft, clear, jelly-like substance called protoplasm, in which lies a nucleus, and (2) certain cavities called vacuoles, which are filled with a clear fluid or cell sap.

Cells from the root of a plant. (X 550).

Cells from the root of a plant. (X 550).

1. Showing youngest cells with thin walls (w), filled with protoplasm and containing nucleus (n), and nucleolus (n').

2. Older cells with thicker walls with vacuoles and cell sap (s).

3. Shows further diminution of protoplasm and increase in cavity (s) in proportion to the growth of the cell wall (w).

Further investigation of the life history of cells, particularly in the early stages of their development, showed that the cell wall, which played so important apart in the original conception of a cell, was not always present, but was formed by the protoplasm in a later stage of growth. The cell sap and other matters were found to occur less commonly, and appeared still later than the cell wall in the lifetime of the vegetable cell; hence it was concluded that they were the outcome of changes due to the activity of the protoplasm, and that this latter was the only essential and formative part of the cell.

Diagram of animal cell (ovum). {Gegenbauer).

Fig. 2. Diagram of animal cell (ovum). {Gegenbauer).

a. Granular protoplasm.

b. Nucleus.

c. Nucleolus.

Liver cell of man, containing fat globules (b) and biliary matters.

Fig. 3. Liver cell of man, containing fat globules (b) and biliary matters. (Cadiat).

Subsequently, from the facts that some vegetable cells in the youngest and most active stage of their growth have no limiting wall, and that most animal cells have none during any part of their life, it was proposed to define a cell as a mass of protoplasm containing a nucleus. But further research showed that the nucleus was not always present. In many cryptogamic plants no nucleus can be found, and in some animal cells, which must be regarded as independent individuals (Protamoeba), there is no nucleus at any part of their lifetime. This would lead us to suppose that a mass of protoplasm capable of manifesting all the phenomena of life would be a sufficient definition. Though this is probably correct in a few cases, the vast majority of cells do contain nuclei. As it is difficult to divest our minds of the connection between the two, it has been proposed to give the name cytode to the non-nucleated forms, which certainly are very exceptional, reserving the term cell for the common nucleated unit. Each part of the cell may now be considered in the order of its importance, viz., protoplasm, nucleus, cell wall, and cell contents.