98 THE NATURE OF LIFE 



a result their ovaries began to develop 

 and they were able to lay eggs. In this 

 connection it may be recalled that Evans 

 has recently announced the discovery of 

 "vitamin X" which is necessary for fer- 

 tility. Rats were given a diet (casein, 

 lard, and cornstarch) on which they 

 grew and developed in a normal manner 

 but in some cases they were completely 

 sterile. The sterility could be cured by 

 the addition of certain substances to the 

 diet, such as lettuce, alfalfa, wheat germs, 

 milk fat, or meat. 



Vitamin X is distinct from the other 

 well known vitamins, such as the fat 

 soluble A and the water soluble B and C, 

 which are necessary for growth and 

 health. Very little is known of their 

 nature, but another, vitamin D (formerly 

 called bios), which is needed for the 

 growth of yeast has recently been ob- 

 tained in crystalline form and we may 

 hope to learn its chemical structure. 



A great many cases are known where^ 

 as in the case of vitamins, small amounts 



