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 (1) 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.

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 a part 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 con

cluded 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.

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 (Protamoba), 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.

1. Protoplasm is commonly seen to be a colorless, pale, milky, semi-translucent substance, more or less altered in appearance by various foreign matters lying in it. These latter also give it a granular appearance, and when dead it commonly exhibits a linear marking or fine network. During life its consistence is nearly fluid, varying with the circumstances in which it is placed, from that of a gum solution to a soft jelly. When living unmolested in its normal medium it seems to flow into various shapes, but this is a living action which does not prove it to be diffluent, for any attempt to investigate it by experiment causes a change in its consistence approaching to rigidity.

As the full comprehension of the function of this substance lies at the root of the greater part of Physiology, the reader is referred for a detailed account of its properties to Chapter III, on Vital Phenomena, where it will be discussed at greater length.

2. The Nucleus.-The majority of independent masses of protoplasm, and all highly organized cells, contain one or more nuclei in their substance. The nucleus is sharply marked off from the protoplasm, and is supposed to be surrounded by a special limiting membrane. Its presence can generally be made much more conspicuous by treating the cell with certain chemical reagents, notably dilute acids and various dyes. The nucleus is able to resist the action of dilute acetic acid better than the remainder of the cell, so that it stands out clearly, when the rest becomes transparent. Many staining agents, such as magenta (one of the aniline dyes), color the nucleus more quickly and deeply than the protoplasm. Although it is accredited with special independent movements that occur under certain circum

stances, compared with the protoplasm it is not very contractile. It appears to be intimately associated with the vital phenomena of the cell, and may be said to control or initiate its most important activity, namely, its division. In the nuclear matrix, which is clear and homogeneous, may often be seen an irregular network, one point of which stands out more clearly, and is called the nucleolus. Remarkable changes in the arrangement of this network are seen in some cells to precede the division of the protaplasm. This is called karyokinesis.

3. The Cell Wall.-It has already been stated that the most active cells, such as are found in the earliest stages in the life of an organism (embryonic cells), have no inclosing membrane or cell wall. But in the more advanced stages of cell life we find this second form of protoplasmic differentiation to be common enough. In animal cells the limiting membrane has never the same importance as the cell wall in vegetable tissues, where some of the principal textures may be traced to a direct modification of the cell wall, still recognizable as such. Whenever such a limiting membrane exists, it is formed by the outer layers of protoplasm undergoing changes so as to become of greater consistence. In the animal tissues the cells form various structures, which are not limiting membranes or cell walls, but rather give the idea of lying between the cells. Hence, in one large group

of tissues, they have been called intercellular substance, while in others they appear as materials specially modified for the furtherance of the functions of the special tissues.

4. Cell Contents.-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 con

FIG. 4.

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

tents 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.

Varieties of Cells.-Great varieties of cells are found in the various mature tissues of the higher animals, all of which have passed through the stage of being a simple nucleated mass of protoplasm in the earlier periods of their development. All cells may then be divided into two chief types, the indifferent and the differentiated.

Under the category of indifferent cells may be placed all such as retain the characters of the first embryonic cells, and have not acquired any special structure or property by which they can be distinguished from the simplest form. Such cells are the only ones in the early stages of the embryo. In the adult tissues they also occur, having various duties to perform.

They

Transverse section of Blastoderm, showing the elements in the earlier stage of the development. A, epiblast; B, mesoblast; C, hypoblast.

are found in the blood and lymph, and scattered throughout the tissues. They are without a cell wall, and have no special contents to mark their function.

Among the differentiated cells we find many special characters, adapting them to certain special duties, for all these cells are modified from the original type and applied to the performance of some special function.

Space prevents even a short enumeration of the varieties of cells met with in the tissues of plants, where they not only carry on the active functions of the organism, but also form the supporting structures.

The differentiation of a cell is accomplished by its protoplasm, which forms new structural parts, and itself sometimes seems to