350 THE MICKOSCOPE. 



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. 2 a, Plate XII. Another change of cell is this : 

 they shoot out processes from certain parts of them, as may be seen at 

 fig. 10 a, Plate XII. ; this may be found also in the choroid plexus, 

 on the inner surface of the sclerotic coat of the eye, or lamina fusca, 

 as it is called. The cylindrical form of cell is found with delicate 

 processes shooting out from the broad end ; these are called ciliated, 

 seen at fig. 2 b, Plate XII., and the cilia are endowed with the power 

 to move spontaneously, having a vibratile motion, intended to urge on 

 the secretions of the part in a particular direction. 



Change of the Nucleus. — The nucleus may undergo a change ; it 

 may be smooth, round, compressed, like the cell to which it belongs ; 

 it may disappear altogether, and the cell which contained it remain. 

 The corpuscles of the blood and the epidermic scales in the last 

 stage of development are examples. The contents may change with 

 the membrane itself. Some cells are filled with a granulous matter, 

 others with pigment, or colouring-matter, as the cells of the choroid of 

 the eye. Others, again, become filled with matters which form the 

 secretions ; the cell-membrane breaking, and pouring out its contents. 

 The cell-membrane may become so changed, as to be of a horny con- 

 sistence, not capable of being acted on by acetic acid, as it could have 

 been before; this is well seen in the last changes of the epidermoid 

 and epithelium cells. 



In some cases the walls of the cell increase in thickness. Under 

 the microscope, some cells appear to be composed of concentric la- 

 minae. In plants this is the common mode of increase in the thick- 

 ness 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, and these canals are named pore- 

 canals. They do not perforate the outer layers, consequently the blind 

 ends are seen through the outer membrane, and were supposed, indeed, 

 to be apertures ; but they are not so. Henle thinks he has found canals 

 in such cells in animals, similar to those in vegetables — in the cartilage 

 of the epiglottis, for instance. Another mode of development is, that 

 the cells may not remain free and independent, but may coalesce with 

 each other. Of this there are two modes : the first is, before coalescence 

 the cell may have attained its full development as a cell ; or secondly, 



