1 82 THE POPULAR SCIENCE MONTHLY 



number of bones or other organs in the human body does not depend 

 upon the size of the man. 



In animals in which the number of organs is constant the con- 

 stituent parts of such organs may vary in number with the size of the 

 organs. Thus in a large Crepidula plana the gill is composed of more 

 than two hundred large filaments, in a dwarf it consists of only fifty or 

 sixty small ones. The liver, sex glands and salivary glands are com- 

 posed of a larger number of lobules in large animals than in small ones, 

 and the size of each lobule is also larger. Evidently the number of 

 such body parts, whether segments, organs, filaments or lobules, depends 

 upon the power of growth and subdivision of each of these parts. In 

 general the more complex any part becomes the less capable it is of sub- 

 division, and so in all highly differentiated animals we find the body 

 parts and organs are constant in number, though variable in size; 

 whereas in lower animals the number of body parts as well as their 

 individual size may vary with the size of the body as a whole. 



Cells are generally recognized to be the ultimate independent units 

 of organic structure and function ; the causes of growth and differentia- 

 tion, of assimilation and dissimilation, of longevity, senescence and re- 

 juvenescence are to be looked for in cells. What is the relation of body 

 size to cell size and cell number ? A large number of investigators have 

 studied this problem in a wide range of animals and plants, and with 

 apparently conflicting results ; nevertheless enough is now known I think 

 to permit a general answer to this question. Just as in the case of 

 body parts and organs, so also with cells, complexity of differentiation 

 and power of division are generally in inverse ratio. In many animals 

 and plants certain types of cells continue to divide throughout life, 

 where other types cease to divide at an early age. In both plants and 

 animals those cells which continue to divide throughout the growing 

 period become more numerous in large organisms than in small ones, 

 but not individually larger ; on the other hand cells which cease to divide 

 at an early stage in the life cycle become individually larger in large 

 animals than in small ones, though in closely related forms their 

 number may remain the same. In short, the size of cells is directly 

 proportional to the rate and duration of growth and inversely propor- 

 tional to the rate of division. It is well known that muscle cells and 

 nerve cells cease to divide at a relatively early age, whereas epithelial 

 and gland cells, mesenchyme, blood and sex cells continue to divide for 

 a longer period, if not throughout life; accordingly, one would expect 

 to find that muscle cells and nerve cells are larger in giants than in 

 dwarfs, but that the other types of cells named would differ in number 

 but not in size — and this is the general result reached by most of the 

 investigators who have worked on this subject (Donaldson, Levi, 



