242 RECENT CYTOLOGY 
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 Ameba 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 
