532 THE MICROSCOPE. 



progeny; the contents of these cells are the cytoblastema 

 to other nucleated cells. 



As an instance where cells are not directly derived from 

 cells, but previously-existing cells exert an influence on 

 those to be formed, we may instance a fractured bone, 

 between the ends of which osseous matter is deposited. 

 We infer from this, that the substance of the bone deter- 

 mines, as it were, the formation of other cells, first into 

 cartilage, and then into bone. (See drawings of these 

 further on.) Generally, however, where a part has to be 

 repaired, it does not seem to determine the generation of 

 a texture similar to itself muscle and skin, for examples. 

 We have an exception to the last observation in the case 

 of nerves, which, if cut across, a substance is formed 

 between the ends which can transmit the nervous in- 

 fluence ; but the ends must not be separated to any great 

 distance, or this will not occur. The same remark applies 

 to bone. Cells may retain an independent existence, 

 although changes may take place in their walls and con- 

 tents, or they may become eventually dissolved and be 

 succeeded by new ones. They may change their form, 

 that is, a globular eell may pass to the compressed form; 

 and this may arise from the difference of the contents to 

 the material outside it, as in the corpuscles of the blood. 

 Flattening of cells may arise from the pressure they exert 

 upon each other, as, for example, in the cells of the epi- 

 dermis and the epithelium. In some cases the cells become 

 so thin, that their thickness cannot be measured. Some- 

 times, where there is but a single layer of cells, flattened, 

 the hexagonal form of cell is assumed; at other times, the 

 polygonal. When a mass of cells compress each other, 

 they take the polygonal form, and have length, breadth, 

 and thickness, as in the fat-cells of ruminating animals; 

 this is readily seen in the fat of beef, but in human fat the 

 round form is maintained. There may be a single layer 

 of cells so arranged side by side, and presenting a columnar 

 or basaltic form; this arrangement is seen in the cells of 

 the intestinal tract, fig. 274, a. Another change of cell is 

 this : they shoot out processes from certain parts of them, 

 as seen at fig. 269, Nos. 10, 11 ; the same occurs also in 

 the choroid plexus ; and on the inner surface of the sclerotic 



