DIFFERENTIATION AND REJUVENATION 151 



the screen (Fig. 22) which showed motor nerve cells 

 of various animals. You will recall that I directed 

 your attention to the fact that the largest animal, the 

 elephant, has the largest cells, and the smallest ani- 

 mals, the rat, the mouse, and the little bat, have the 

 smallest ones. But let me point out to you that the 

 question of the size of cells is exceeding complex, and 

 that in studying it we have to exercise a great deal of 

 caution. We know that, with the exception of the 

 nerve cells and to a minor degree with the exception 

 of the muscle fibres, the cells in each animal are more 

 or less uniform constants in size. The cells of dif- 

 ferent organs differ somewhat from one another. A 

 single organ may have in its different parts typical 

 sizes of cells, but each of these kinds of cells has its 

 definite dimensions. When one animal is larger than 

 another, it has more cells. Now it is a very important 

 fact for us that animals have a more or less constant 

 size of their cells. They do not differ from one an- 

 other by a difference in the size of their cells ; the 

 bigness of an animal does not depend upon the size, 

 but upon the number, of its cells. We can, therefore, 

 in studying the changes of size, to which I shall next 

 direct your attention, omit altogether these details, 

 and speak of the cells in a general way safely as hav- 

 ing a certain uniform or standard size. This will save 

 us a great deal of time, for we learn, as we study cells, 

 that their size increases with the age of the animal. 

 Since the animal, when it is young, has cells with a 

 smalji amount; of protoplasm, the increase of proto- 



