==The Cell Doctrine==.—­The cell doctrine is, in brief, the theory that the bodies of animals and plants are built up entirely of minute elementary units, more or less independent of each other, and all capable of growth and multiplication.  This doctrine is commonly regarded as being inaugurated in 1839 by Schwann.  Long before this, however, many microscopists had seen that the bodies of plants are made up of elementary units.  In describing the bark of a tree in 1665, Robert Hooke had stated that it was composed of little boxes or cells, and regarded it as a sort of honeycomb structure with its cells filled with air.  The term cell quite aptly describes the compartments of such a structure, as can be seen by a glance at Fig. 7, and this term has been retained even till to-day in spite of the fact that its original significance has entirely disappeared.  During the last century not a few naturalists observed and described these little vesicles, always regarding them as little spaces and never looking upon them as having any significance in the activities of plants.  In one or two instances similar bodies were noticed in animals, although no connection was drawn between them and the cells of plants.  In the early part of the century observations upon various kinds of animals and plant tissues multiplied, and many microscopists independently announced the discovery of similar small corpuscular bodies.  Finally, in 1839, these observations were combined together by Schwann into one general theory.  According to the cell doctrine then formulated, the parts of all animals and plants are either composed of cells or of material derived from cells.  The bark, the wood, the roots, the leaves of plants are all composed of little vesicles similar to those already described under the name of cells.  In animals the cellular structure is not so easy to make out; but here too the muscle, the bone, the nerve, the gland are all made up of similar vesicles or of material made from them.  The cells are of wonderfully different shapes and widely different sizes, but in general structure they are alike.  These cells, thus found in animals and plants alike, formed the first connecting link between animals and plants.  This discovery was like that of our supposed supramundane observer when he first found the human being that brought into connection the widely different cities in the various parts of the world.

[Illustration:  FIG. 7.—­A bit of bark showing cellular structure.]

Schwann and his immediate followers, while recognizing that the bodies of animals and plants were composed of cells, were at a loss to explain how these cells arose.  The belief held at first was that there existed in the bodies of animals and plants a structureless substance which formed the basis out of which the cells develop, in somewhat the same way that crystals arise from a mother liquid.  This supposed substance Schwann called the cytoblastema, and he thought