328 SCIENCE FROM AN EASY CHAIR 



In order to appreciate this matter it is necessary to 

 know the chief facts about the ordinary process of repro- 

 duction in animals and plants. All animals and plants 

 are built up of minute particles of living matter called 

 ''cells" (see p. 170). Really, these are not cells, or hollow 

 boxes, or cases. We use the word " cell " for the contents 

 of a cell. Each is a droplet of protoplasm or living matter 

 lying in a small or large envelope or case of dead matter 

 which it has produced around itself (Fig. 61). Observers 

 using their microscopes saw at first only the case, and 

 called it a " cell," and the word " cell " is now used almost 

 universally for the soft stuff within the cell (see p. 173). 

 Each soft cell of " plasm " or " protoplasm " has a very 

 special structure. The existence in it of a central kernel, 

 or " nucleus " of peculiarly active substance, is the most 

 obvious feature. These " cells " are so small (for instance, 

 those which build up the human body) that from one to 

 two thousand could be placed side by side on a line an 

 inch long. They are the " units " which make up the body 

 of an animal or plant, just as bricks and planks and rods 

 make up a building constructed by human contrivance. 

 Two most important things about them are first, that 

 each is always the seat of chemical activity, absorbing 

 liquid material, changing it and either fixing it or throw- 

 ing it out in a new chemical condition ; and, second, that 

 as a result each cell grows, and after a very little growth 

 divides into two. This " dividing into two " is immensely 

 important, for in this way the number of cells forming a 

 very young or small animal or plant is increased from 

 a few thousands to many millions whilst the organism 

 grows. And not only that, but we find on tracing the 

 young animal or plant back to its beginning as an indi- 

 vidual that it actually started as a single cell. The germ 

 of every living thing, then, is a single nucleated particle 

 of protoplasm a cell which we call the " egg-cell," because 



