xi THE ELEMENTS OF STRUCTURE 201 



cells that build up the body of an animal may be briefly 

 noticed. The first is that in some cases there is great 

 definiteness in the number and position of the cells 

 building up the body ; thus in the Rotifer Hydatina 

 senta there are 959 cells in the body, and each has its 

 proper place. The second fact is that there is often 

 remarkable definiteness in corresponding cells in different 

 animals, so that those of one type could not be confused 

 with those of another. Thus the ciliated epithelial cells 

 lining the windpipe of a horse are different from those 

 in the same place in a dog. This is called " specificity," 

 and it means that each particular type has an individuality 

 or idiosyncrasy of its own, and that this is demonstrable 

 in great detail. 



The growth of all multicellular animals depends upon 

 a multiplication of the component cells. Like organisms, 

 cells have definite limits of growth which they rarely 

 exceed ; giants among the units are rare. When the 

 limit of growth is reached the cell divides. 



The necessity for this division, as indicated by Spencer, 

 Leuckart, and James, has been already referred to. But 

 we wish to dwell on it. If you take a round lump of 

 dough weighing an ounce, another of two ounces, a third 

 of four ounces, you obviously have three masses succes- 

 sively doubled, but in doubling the mass you have not 

 doubled the surface. The mass increases as the cube, 

 the surface only as the square of the radius. Suppose 

 these lumps alive, the second has twice as much living 

 matter as the first, but not twice the surface. Yet it is 

 through the surface that the living matter is fed, aerated, 

 and purified. The unit will therefore get into physio- 

 logical difficulties as it grows bigger, because its increase 

 of surface does not keep pace with its increase of mass. 

 Its waste tends to exceed its repair, its expenditure gains 

 on its income. What are the alternatives ? It may go 

 on growing and die (but this is not likely) ; it may cease 

 growing at the fit limit ; it may greatly increase its 

 surface by outflowing processes (which thus may be 

 regarded as life-saving) ; or it may divide. The last is 

 the usual course. When the unit has grown as large as 



