Every plant, every animal, commences its existence as a single cell. An organism which is permanently unicellular divides. Each of the separate cells into which it divides is a new individual. Higher plants set aside certain cells as ovules, which in due course, after conjugation with pollen grains, grow into plants. In the same way the ova of animals, by repeated cell division, reproduce the species. The individual commences as a single cell. Its complicated body, composed of various organs and various tissues, is formed by the multiplication of cells. Each of the innumerable cells of which it is composed has the structure, and may therefore be presumed capable of performing all the various functions, of a unicellular organism. But it does not follow that the cells retain their individuality. Even unicellular plants (_e.g._, the extraordinary vinegar and tan fungi, myxomycetes) may for a time merge their individuality in a common mass formed by the aggregation of many cells.
The cells of higher plants are not always, or even generally, anatomically distinct. Their protoplasm, the essential part of every cell, is united with the protoplasm of neighbouring cells by threads which traverse the cell-walls. The cells of the connective tissues of animals are united into a web, or syncytium. This is especially noticeable during early stages of growth. Nerve-cells are connected together by conducting filaments (neuro-fibrillæ). It is possible that nerve-cells and the muscle-fibres which they innervate are from the beginning united by nerve-filaments—that the nerve-cell and muscle-cell grow apart without severing this thread-like connection. Certain anatomists regard the nerve strand which connects a cell in the central nervous system with a number of muscle-fibres, placed, it may be, at a great distance from the nerve-cell, as the bridge which has never been broken in the process of cell division and displacement, which made one primitive cell into a nerve-cell and a group of muscle-cells. Muscle-fibres are not separate cells, but cell complexes. Each muscle-fibre contains scores, in some cases hundreds, of nuclei (Fig. 16). It is a cylinder, perhaps 2 inches long, in which cell division is incomplete. Tendons are bundles of exceedingly slender fibres which lie side by side, like silk threads in a skein. The row of cells which gives rise to a tendon undergoes incomplete cell division. Their nuclei divide, and a small quantity of soft body-substance is set apart for each nucleus. The rest of the mass consists of fused cells. It constitutes a continuous rod, which becomes fibrillated as it grows. Vegetable cells are separated by cell-walls. Animal cells tend to develop intermediate partitions; but the partitions are so thick that they can no longer be described as walls. In cartilage the cell-bodies are embedded in a great mass of intercellular substance, or matrix. In this intercellular substance elaborate developments may take place. Elastic fibres may make their appearance in it to form elastic cartilage, as in the case of the epiglottis. In these various instances, although it is perfectly true that tissues are formed by cell division, the cells are not, strictly speaking, separate units. They are not completely divided one from another. It is impossible to recognize their anatomical boundaries.