666 THE MICKOSCOPF. 



regeneration of tissue is proceeding, there is found a 

 larger number of granules. 



According to some histologists, cells may be formed in 

 cytoblastema independent of any pre-existing cells ; this 

 is cited as an instance of that mysterious agency desig- 

 nated spontaneous generation. There are cells which, so 

 far as we know, after full development, undergo no further 

 metamorphosis ; such as those of the epithelium, the blood 

 corpuscles, &c. 



As an instance where previously-existing cells exert an 

 influence on those about to be formed, we may adduce a 

 fractured bone, between the ends of which osseous matter 

 is deposited. We infer from this, that the substance of the 

 bone determines, as it were, the formation of other cells, 

 first into cartilage, and then into bone. Generally, how- 

 ever, where a part has to be repaired, it does not seem to 

 determine the generation of a texture similar to itself 

 for example, muscle and skin. 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 transmits 

 the nervous influence, but the ends must not be separated 

 to any great distance, or this will not occur. The 

 same remark applies to bone. 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. 303 a. Another 

 change of cell is this : they shoot out processes from 

 certain parts of them, as seen in fig. 300, Nos. 10, 11; 

 this kind of formation occurs on the inner surface of 

 the sclerotic coat of the eye. The cylindrical form of cell 

 is found with delicate processes shooting out from the 

 broad end ; these are called ciliated, fig. 303 d, and the 

 cilia having a vibratile motion urge on the secretions of 

 the part in a particular direction. 



In some cases the walls of the cell increase in thickness 

 (fig. 300, No. 5). Under the microscope, some cells 

 appear to be composed of concentric laminae. In plants 

 this is the common mode of increase in the thickness of 

 the cell, but the deposit does not take place entirely 

 around, but only here and there, so that vacant spaces 

 are left which form canals, and may become branched : 



