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AUTHOR’S PREFACE. In the above-mentioned investigations of Henle, Turpin, and Dumortier, the resemblance which the animal tissues examined (epithelium and the liver or yelk of snails) bore to plants, lay, in the first place, in the circumstance, that their elementary particles grew without vessels, and in part, free in a fluid, or even inclosed in another cell; and in the second place, in that these elementary particles exhibiting a non-vascular growth, were furnished with a peculiar wall, like the cells of plants. When this coincidence was furnished, we were entitled to arrange these cells as near to the vegetable cells as the different kinds of animal cells, for instance, germinal vesicles, blood-corpuscles, and fat-cells, stand together, when regarded as different species comprised under the natural-history idea of cells.

The state of the matter, therefore, when I commenced my researches was as follows: The elementary particles of organised bodies presented the greatest variety of form; there was a resemblance between many of them, and, according to the greater or lesser degree of similarity, a group of fibres, of cells, of globules, and so on, might be distinguished, and in each of these divisions again there were different forms. As the cells taken collectively differed from the fibres, so also,only in a less degree, must the separate kinds of cells differ from each other, and the different kinds of fibres from each other. All those forms seemed to have nothing else in common, save that they grew by the addition of new molecules between those already existing, that they were living elements. So long as the epithelium-cells were regarded as a secretion of the organised substance, they could never, in that sense, be classed with the living elementary particles. There seemed to be no general rule with respect to the mode in which the molecules were joined together to form the living particles; here they united into one kind of cells, there into another, and at a third spot into a fibre, and so on. The principle of development appeared to be altogether different for such particles as differed