Regarding protoplasm as the essential living part of the cell, under this heading will come only those extraneous matters which are the outcome of protoplasmic activity.

The cell contents which are present with such constancy and in such variety in vegetable cells, form in them an all-important part; but in most animal cells the contents do not occupy such a striking position.

No doubt animal protoplasm is quite as capable as that of vegetables of making out of its own substance, or the nutriment supplied to it, a great variety of materials, but these are seldom stored in such large quantities in animal cells as in those of plants.

In the cells of some kinds of animal textures, particularly that called Connective Tissue, we commonly find large quantities of fat formed and accumulated to such a degree in the cell that the protoplasm can be no longer recognized as such. Its remnant is devoted to forming a limiting membrane for the fatty contents, so that the cell is converted into an oil vesicle, and here what may be termed the contents become the most important part of the cell. In various glandular cells, as will be seen hereafter, different substances are made and stored up temporarily in the protoplasm. These may be seen as bright refracting granules, which are subsequently discharged in the secretion of the gland.

In other cells (liver) nutrient material allied to starch may be deposited in considerable quantity, just as starch is stored in certain cells of plants, but owing to the greater and more constant activity of animals, the amount laid by never attains anything like that found in the store textures of vegetables, where the result of an entire summer's active work is put by as a provision for the next winter and the fresh burst of energy which follows it in the spring.

But while the above are all more or less temporary contents of cells, we have an example of a permanent deposit in them, viz., pigment; this substance is formed by the protoplasm in various parts, and has a special physiological use. Thus in the tissue behind the retina - or nerve layer of the eyeball - the cells are filled with granules of a pigmented substance, which absorbs the light falling upon it, and thus prevents the reflections which would interfere with the clearness of sight.

It also occurs in the skin of the negro and other races, and in that of the frog and other animals, but in these its function is not fully known.

Cell from connective tissue containing large fat globule (a), and showing protoplasm (p), and nucleus (n) {m), membrane.

Fig. 4. Cell from connective tissue containing large fat globule (a), and showing protoplasm (p), and nucleus (n) {m), membrane. (Ranvier).