Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/30

INTRODUCTION. like a watch-glass upon a watch. It is at this time so delicate that it dissolves in distilled water in a few minutes. It gradually expands, becomes more consistent, and at length so large, that the cytoblast appears only as a small body inclosed in one of the side walls. The portion of the cell-wall which covers the cytoblast on the inner side, is, however, extremely delicate and gelatinous, and only in rare instances to be observed; it soon undergoes absorption together with the cytoblast, which like-wise becomes absorbed in the fully-developed cell. The cytoblasts are formed free within a cell, in a mass of mucus-granules, and the young cells lie also free in the parent cell, and assume, as they become flattened against each other, the polyhedral form. Subsequently the parent cell becomes absorbed. (See a delineation of young cells within parent cells, plate I, fig. 2, b, b,b.) It cannot at present be stated with certainty that the formation of new cells always takes place from a cystoblast, and always within the existing cells, for the Cryptogamia have not as yet been examined in this respect, nor has Schleiden yet expressed his views in reference to the Cambium. Moreover, according to Mirbel, a formation of new cells on the outside of the previous ones takes place in the intercellular canals and on the surface of the plant in the Phanerogamia. (See Mirbel on “Marchantia,’ in Annales du Musée, 1, 55; and the counter-observations of Schleiden, Müller’s Archiv, 1838, p. 161.) A mode of formation of new cells, different from the above de- scribed, is exhibited in the multiplication of cells by division of the existing ones ; in this case partition-walls grow across the old cell, if, as Schleiden supposes, this be not an illusion, inasmuch as the young cells might escape observation in consequence of their transparency, and at a later stage, their line of contact would be regarded as the partition wall of the parent cell.

The expansion of the cell when formed, is, either regular on all sides, in which case it remains globular, or it becomes polyhedral from flattening against the neighbouring cells, or it is irre- gular from the cell growing more vigorously in one or in several directions. What was formerly called the fibrous tissue, which contains remarkably elongated cells, is formed in this manner. These fibres also become branched, when different points of the cell-wall expand in different directions. This expansion of