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ON CELLS, ETC. form presented by the cells, for they may be flattened even to the total disappearance of the cavity, or elongated into cylinders and fibres. From these circumstances, many of the cells which now come before us for consideration, have been described as mere globules, or granules, terms which do not express their true signification, and even when they were spoken of as cells, or cells furnished with a nucleus, the description rested only upon a slight analogy, since but very few of them (for example, the pigment-cells), were proved to be actually hollow cells. But—as the precise signification of the nucleus is unknown, and as the cell-membrane is not proved to be anything essential to those cells (and this follows from their accordance with vegetable cells), upon the analogy with which the proof of the cellular nature of the rest of the globules provided with a nucleus will be based,— there is no contradiction involved in the supposition that a nucleus may be contained in a solid globule as well as in a cell.

From the difficulties of this investigation above detailed, it will be seen that a given object may really be a cell, when even the common characteristics of that structure, namely, the perceptibility of the cell-membrane, and the flowing out of the cell-contents, cannot be brought under observation. The possibility that an object may be a cell, does not, however, advance us much; the presence of positive characteristics 1s necessary in order to enable us to regard it as such. In many instances these difficulties do not present themselves, and the cellular nature of the object is immediately recognized; in others, the impediments are not so great but that the distinction between cell-membrane and cell-contents is at least indicated, and in such cases other circumstances may advance that supposition to a certainty. The most important and abundant proof as to the existence of a cell is the presence or absence of the nucleus. Its sharp outline and dark colour render it in most instances easily perceptible; its characteristic figure, especially when it encloses nucleoli, and remarkable position in the globule under examination, (being within it, but eccentrical, and separated from the surface only by the thickness of the assumed cell-wail,) all combine to prove it the cell-nucleus, and render its analogy with the nucleus of the young cells contained in cartilage, and with those of vegetables, as also the analogy between the