Page:Popular Science Monthly Volume 83.djvu/186

182 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 constituent 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 composed 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 subdivision, 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 differentiation, of assimilation and dissimilation, of longevity, senescence and rejuvenescence 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 proportional 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,