The bodies of all the higher animals are composed of a great variety of parts, different in their structure and action, and yet so beautifully adapted to each other as to act in perfect harmony.

In the lowest forms of vegetable life we find a single cell making up the whole fabric. This cell, grows from its germ, absorbs and assimilates nutriment, converts a part of it into its own cell-wall, secretes another portion into its cavity, and from a third, produces the reproductive germs that are to continue its race. Having completed the germs, it bursts and sets them free. Each one of these germs is capable of going through the same set of operations.

In the higher forms of vegetable life we find a multiplication of similar cells, among which these operations are distributed, thus producing by the labor of all a more complete and permanent effect.

At the extremities of the roots of plants we find succulent bodies made up of soft cells, known by the name of spongioles. These perform the absorption of nutritious fluid, which is conveyed by the vessels of the stem and branches to the leaves, and there in the cells, which make up the parenchyma of those organs, undergoes a change. The watery ascending sap is converted into thick glutinous latex, which like the blood of animals contains the material for the production of new tissue and the elements of the various secretions. This process of conversion comprises not only the exhalation of superfluous liquid, the action of light, and the interchange of gaseous ingredients between the sap and air, but a new molecular arrangement of the particles of sap, by which new products are generated. This process, which is such an immense step towards the production of living tissue from the crude material, is called assimilation.

As the latex descends in its proper vessels through the stem, it yields up to the growing parts the nutrition they severally require. Beside the ordinary tissue, of which most of the fabric is composed, in the growth of which the process of nutrition is considered as consisting, there are groups of cells, which separate peculiar products from the sap such as oil, starch, resin, etc, which are stored up against the time they are demanded. These are said to perform the act of secretion. All the cells by which the permanent fabric is provided for, have as individuals but a very transitory life. The absorbents are continually renewed, some dying, and others forming the solid texture of the root. In the short duration of the assimilating cells, we have a convincing proof in the fall of the leaf and the opening buds. The secreting cells undergo a like transitory duration.

The starting point both in the animal and the plant is the same. The embryo of the animal up to a certain grade of its development, consists, like that of the plant, of nothing else than an aggregation of cells. Among the higher class of animals, however, a large proportion of the fabric consists of tissues in which no distinct trace of cellular origin is apparent, and yet when we subject them to a close analysis, and examine them not only in their complete state but in their development, we find they are reduced to the same category with the tissues of the plants and lower animals. There are tissues peculiar to animals, and these we find referable to the plastic fluid prepared by the assimilating cells, and set free by their rupture. In plants, the tissues principally concerned in the vital operations retain their cellular form. We also see distinct groups of cells in the bodies of animals, which have not only the functions of absorption, assimilation, respiration, secretion, and reproduction, which we also find in plants, but those of muscular contraction and nervous action, which they alone perform.

The cell originates from a reproductive granule, previously formed by some other cell; this granule attracts to itself, assimilates and organises the particles of the nutrient fluid in its neighborhood, and converts some of them into the substance of the cell-wall, and draws others into the cavity of the cell. In this way the cell gradually increases in size, and while it approaches its term of life, makes preparation for its renewal, by the development of reproductive granules in its interior, which may become the germs of new cells, when set free from the cavity of the parent by the rupture of the cell-wall. In the chyle, lymph, and blood, we find floating cells, called the chyle and lymph-corpuscles and colorless corpuscles of the blood, having no single nucleus, but several scattered particles, each of which is a reproductive granule. These, when set free by the bursting or liquefaction of the walls, float in the current of fluid, and are in their turn developed into cells. In general however, the cells of animal tissue are furnished with a nucleus. The nucleus seems to be the chief instrument in the function of the cell. In some cells this function is restricted to the attraction of certain constituents, by which its cavity is filled. These constituents may be of a fluid nature readily passing into decomposition, such as the glandular structure, or they may give solidity to the texture. Thus the cells of the epidermis are strengthened by a deposit of a horny nature, those of shell by the deposit of carbonate of lime, and those of the bones and teeth by mixture of earthy and mineral matter. These cells do not generally reproduce themselves, but successive crops of them are formed as fast as they are required from other sources.

Cells are often elongated, and their cavities occupied by internal deposit, so that they may be mistaken for solid fibres, or the boundary of the cells may be lost by their coalescence with each other. The character of the cell may be completely changed by a solution in its wall in one or more spots so that its cavity is laid open and coalesces with some other. Thus, by the disappearance of the partition between the cells may be formed a tube, and this may coalesce with others to form a capillary net-work for the circulation of blood.