Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/64

ON CELLS AS THE BASIS globules under examination, in which it lies, and those cells, consequently the existence of a spherical cell-membrane in the globules, extremely probable. More than nine tenths of the globules in question present such a nucleus; in many the special cell-membrane is indubitable, in most it is more or less distinct. Under such circumstances, we may be permitted to conclude that all those globules which present a nucleus of the characteristic form and position, have also a cell-membrane, although, from the causes before specified, it may not be perceptible. The different tissues will also afford us many instances of other circumstances which tend to prove the existence of an actual cell-membrane. An example of what is referred to would be afforded by an instance, in which a certain corpuscle (furnished with a nucleus), about the cellular nature of which a doubt existed, could be proved to be only a stage of development, or modification in form, of an indubitable cell. The cell-nuclei and their distance from each other when scattered in a tissue, also serve as indications, when the outlines of the cells have to be sought for. They likewise serve to guide conjecture as to the earlier existence of separate cells, in instances where they have coalesced in the progress of develop- ment. When a globule does not exhibit a nucleus during any one of the stages of its development, it is either not a cell, or may at least be preliminarily rejected, if there be no other circumstances to prove it such.’ Fortunately, these cells devoid of nuclei are rare.

In addition, however, to the cellular nature of the elementary structures of animal tissues, there are yet other points of accordance between them and the cells of plants, which may generally be shown in the progress of their development, and which give increased weight to the evidence tending to prove that these elementary structures are cells. The exceedingly frequent, if not absolutely universal presence of the nucleus, even in the latest formed cells, proves its great importance for their existence. We cannot, it is true, at present assert that, with regard to all cells furnished with a nucleus, the latter is universally the primary and the cell the secondary formation, that is to say, that in every instance the cell is formed around the previously existing nucleus. It is probable, however, that such is the case generally, for we not only meet