Page:Microscopicial researchers - Theodor Schwann - English Translation - 1947.pdf/198

172 SURVEY OF CELL-LIFE. impregnated with the nutritive material for the other kind of cells.

According to Schleiden, new cells are never formed in the intercellular substance in plants; in animals, on the contrary, a generation of cells within cells is the less frequent mode, but this does occur, and in such a way, that a threefold or fourfold generation may take place in succession within one cell. Thus, according to R. Wagner’s observations (see the Supple- ment), the Graafian vesicle appears to be an elementary cell; the ovum is developed within it in like manner as an elementary cell; within this, again, according at least to observations made upon the bird’s egg, cells are generated, some of which contain young cells. It appears also, that a formation of true cartilage-cells can sometimes take place within those which already exist, and that young cells (fat-cells?) may be generated within them again. Several such examples might be brought forward; but by far the greater portion of the cells of cartilage are formed in the cytoblastema on the outside of the cells already present, and we never meet with a generation of cells within cells in the case of fibre, muscle, or nerve.

General phenomena of the formation of cells. Round corpuscles make their appearance after a certain time in the cytoblastema which, in the first instance, is structureless or minutely granulous. These bodies may either be cells in their earliest condition (and some may be recognized even at this stage), that is, hollow vesicles furnished with a peculiar structureless wall, cells without nuclei, or they may be cell-nuclei or the rudiments of cell-nuclei, round which cells will afterwards be formed.

The cells without nuclei, or, more correctly, the cells in which no nuclei have as yet been observed, occur only in the lower plants, and are also rare in animals. For the present, however, the following must be regarded as such, viz.: the young cells contained within others in the chorda dorsalis (see p. 13), the cells of the yelk-substance in the bird’s egg (p. 50), the cells in the mucous layer of the ger- minal membrane of the bird’s egg (p. 60), and some cells of the crystalline lens (p.88). Pl. I, fig. 10, c, represents one