THE CELL AND PROTOPLASM. 



69 



FIG. 16. Frog's blood : a and b are the 

 cells; c is the liquid. 



structure is more evident. Thus it happens in 

 adult animals that the cells which are large and 

 clear at first, become less and less evident, until 

 the adult tissue r^-- 



seems sometimes to ;.vv;n-l- : -- : - ~ -"- - 



be composed most- 

 ly of what we have 

 called formed ma- z-'-^*_ 

 terial. 



It must not be 

 imagined, however, 

 that a very rigid 

 line can be drawn 

 between the cell 

 itself and the ma- 

 terial it forms. 

 The formed material is in many cases simply a 

 thickened cell wall, and this we commonly regard 

 as part of the cell. In many cases the formed 

 material is simply the old 

 dead cell walls from which 

 the living substance has 

 been withdrawn (Fig. 14). 

 In other cases the cell 

 substance acquires peculiar 

 functions, so that what 

 seems to be the formed 

 material is really a modified 

 cell body and is still active 

 and alive. Such is the case 

 in the muscle. In other 

 cases the formed material 

 appears to be manufactured 

 within the cell and secret- 

 ed, as in the case of bone. No sharp lines can be 

 drawn, however, between the various types. But 





FIG. 17. A bit of bone, 

 showing the cells imbedded 

 in the bony matter. 



