THE CELL 225 



especially on the side turned to the north, and in 

 similar shady situations, consists, as a rule, of great 

 numbers of minute Pleurococcus plants, although the 

 size of a single specimen may be represented by a 

 diameter of little more than the two-thousandth part 

 of an inch. 



We are more particularly concerned, however, with 

 the higher animals and plants, the bodies of which are 

 built up of a great number of separate cells. Some 

 of these cells may be modified in various ways, but they 

 all conform, at least in the youthful condition, to types 

 not far removed from those of Amoeba and Pleurococcus 

 respectively. Certain parts of these higher organisms, 

 indeed, such as the bones of vertebrate animals and 

 the wood of trees, do not consist solely of living cells, 

 but are composed to a great extent of dead material 

 excreted or built up by the activity of living cells. 

 These latter have, then, either ceased to live, or they 

 may continue to exist in the interstices of the hard 

 skeletal framework. 



New cells come into existence in only one way 

 namely, by a process of division which takes place in a 

 pre-existing cell. In comparatively rare cases a cell 

 may give off a small bud which forthwith develops 

 into a new cell like the old one. In such a case we may 

 speak of the cell which gives off the bud as the mother- 

 cell, and of the cell into which the bud develops as 

 the daughter-cell. But by far the most frequent 

 method of cell-reproduction, and the only one which 

 is characteristic of the higher animals and plants, takes 

 place by the equal division of an old cell into two 



15 



