THE CELLULAR CHANGES OF AGE 65 



research, and if we understood fully the nature of 

 differentiation and the cause of it, we should have 

 probably got far along towards the solution of the 

 final problem of the nature of life itself. 



The size of animals deserves a few moments of our 

 time, for it is intimately connected with our problem 

 of growth and differentiation. Cells do not differ 

 greatly from one another in size. The range of their 

 dimensions is very limited. This is particularly true 

 of the cells of any given individual animal. Recent 

 careful investigations have been made upon the rela- 

 tion of the size of cells to the size of animals, and it 

 has been found that animals are not larger, one than 

 another, because their cells are larger, but because 

 they have more of them. 1 This statement must 

 be understood with certain necessary reservations. 

 There are some kinds of animals, like the star-fish, 

 which have very small cells ; others, like frogs and 

 toads, which have large cells ; so that a star-fish of 

 the same bulk as a given frog would contain a great 

 many more cells. Our statement is true of allied 

 animals. For example, a large frog differs from a 

 small frog or a large dog from a small dog by the 

 number of the cells. An important exception to this 

 law is offered for our consideration by the cells of 

 the central nervous system, the nerve cells properly 



1 G. Levi, " Vergleichende Untersuchungen ueber die GrOsse der Zellen," 

 Verhandl. Anat. Ges., xix., 156-158. 



G. Levi, " Studi sulla Grandezza delle ce\\u\e," AreAivio ital. anat. embriol., 

 v., pp. 291-358. This paper is important and suggestive. 

 5 



