INTRODUCTION 15 



a pine one hundred feet high. The cells of the mouse are not 

 very much smaller than those of the elephant. 



The increase in the number of cells, the increase in the size 

 of the individual cells, and their differentiation require the 

 construction of additional protoplasm, the construction of cell 

 walls, the construction of the materials of bones, of shells, cara- 

 paces, hair, finger nails, and so on. These are made from the 

 food materials which the organism uses. The beef and bread 

 and butter we eat today becomes living protoplasm or bone or 

 hair tomorrow. This is a complex process. The food must first 

 be digested, broken up into simpler parts in somewhat the same 

 way that the construction of a new building from an old one re- 

 quires that the old one be first torn to pieces. The process of 

 digestion deserves more attention than we can give it here. How 

 the chemical changes involved in digestion are accomplished 

 was a mystery until it was discovered that those catalytic agents, 

 the enzymes, make them possible at the relatively low tempera- 

 tures and slight acidities or alkalinities found within the bodies 

 of living organisms. Mr. Pecksniff, in Martin Chuzzlewit, ap- 

 preciated this remarkable discovery. "The process of digestion, 

 as I have been informed by anatomical friends, is one of the 

 most wonderful works of nature. I do not know how it may be 

 with others, but it is a great satisfaction for me to know, when 

 regaling on my humble fare that I am putting in motion the 

 most beautiful machinery with which we have any acquaintance. 

 I really feel at such times as if I was doing a public service. 

 When I have wound myself up, if I may employ such a term," 

 said Mr. Pecksniff, with exquisite tenderness, "and know that I 

 am Going I feel that in the lesson afforded by the works within 

 me I am a Benefactor to my Kind." 



The products formed by the digestion of the food are built 

 up into the complex chemical compounds which constitute the 

 protoplasm and other constituents of the body. Body construc- 

 tion from digested food by the amoeba or the slime mold which 

 have no cell walls, no bones, nor hair, nor teeth, but only pro- 



