30 THE BODY AT WORK 



is the most living substance. The substance which is most 

 alive always presents itself to us as an imperfectly transparent, 

 viscous material, which proves on analysis to contain a large 

 quantity of certain proteins mixed with various organic and 

 inorganic compounds. Protoplasm is organized into, or dis- 

 tributed amongst, cells, which in any given tissue present a 

 fairly uniform size. What determines the size of cells ? 

 Speaking generally, cells are small say about 0-01 milli- 

 metre in diameter. In early stages of growth, cell division 

 occurs as soon as the cell attains to something like this size. 

 It would seem that when nutriment is abundant cells add to 

 their protoplasm more than they lose. Having attained certain 

 dimensions at which the conditions most satisfactory for cell 

 life reach their limit, cell division occurs. The big drop falls 

 into two smaller drops, each of which grows more rapidly than 

 the big one was growing at the time when it began to divide. 

 But if there be an optimum size for nutritive purposes, this 

 limit is suspended in many cases, and for various reasons. 

 Take the ovum itself as an example. It is vastly bigger than 

 the cells into which it divides. The yolk of a hen's egg is, 

 when first formed, a single cell. By the time the egg is laid 

 cell division has already set in. In the embryo there are 

 cells which surpass the average dimensions the unexplained 

 " gia,nt cells " which appear in the liver as soon as it can be 

 recognized as such (c.f. p. 65). These disappear from the liver, 

 but are for a time evident in the spleen. The large cells found 

 in the marrow of bone, some with a great single nucleus, others 

 containing a bunch of separate nuclei, also show that there is no 

 fixed limit of size. It is generally considered that the giant cells 

 of marrow or, at any rate, those which are multinucleated 

 are leucocytes which are engaged in scooping out the bone ; 

 consuming the hard tissue on the inner surface of the hollow 

 cylinder in order that, by deposition of new material on the 

 outside of the cylinder, the size of the whole bone may be 

 increased leucocytes battening on bone which, owing to 

 interference with its blood- supply, is breaking down. They 

 have not time to divide. Nourishment is superabundant. 

 Although much too large for a vigorous standard of cell life, 

 they continue to grow, putting off the duty of cell division 

 until the supply of nutritious food begins to run short. 



