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AUTHOR’S PREFACE. in their physiological signification; and the diversity in the laws which it was necessary to assume in the development of a cell and a fibre, was also, only in a less degree, necessarily assumed between the different kinds of cells and the different sorts of fibres. Cells, fibres, &c. were therefore merely natural-history ideas, and no conclusion could be drawn from the mode of development of one kind of cell as to that of any other kind; and, in fact, no such deductions were made, although we were acquainted with some important points in the process of development of certain kinds of cells; for example, the blood-corpuscle (see p. 67 of this Treatise), and the ovum (see the Supplement, p. 217). Although the investigations quoted above determined the important fact of the non-vascular growth, they did not thereby effect any change in our views. The idea of proving the similarity of the principle of development for elementary particles which were physiologically different, by a comparison of animal cells with those of vegetables, was not contained in those researches, and with these, therefore, the investigators before mentioned might well come to a stand-still.

The discoveries of Schleiden made us more accurately acquainted with the process of development in the cells of plants. This process contained sufficient characteristic data to render a comparison of the animal cells in reference to a similar principle of development practicable. In this sense I compared the cells of cartilage and of the chorda dorsalis with vegetable cells, and found the most complete accordance. The discovery, upon which my inquiry was based, immediately lay in the perception of the principle contained in the proposition, that two elementary particles, physiologically different, may be developed in the same manner. For it follows, from the foregoing, that if we maintain the accordance of two kinds of cells in this sense, we are compelled to assume the same principle of development for all elementary particles, however dissimilar they may be, because the distinction between the other