NUTRITION. 369 



had then resumed growth, as a result of changes in the dietary, has 

 enabled us to determine to what degree this treatment had affected 

 the physiological life of the individual. Donaldson states, in his 

 monograph on "The Rat," that "the menopause in females of this 

 species commonly appears at an age of 15 to 18 months, but King 

 reports a female 22 months old — crossed with a male of like age — 

 giving birth to a litter of one." Our own records show females which 

 were stunted to the ages of 538, 380, 396, and 425 days respectively, 

 giving birth to litters of young at 28 months (3 young), 25 months (2 

 young), 22 months (8 young), and 20| months (10 young). This indi- 

 cates that the sexual cycles have merely been delayed by retardation 

 of growth instead of disappearing at the end of the usual chronological 

 period at which fertility is supposed to be lost by albino rats. 



Recently we have applied to the growth of chickens some of the 

 dietary experiences gained in the past few years in our studies upon 

 the albino rat. Although the statements in the literature as to the 

 possibility of conducting such experiments were far from encour- 

 aging, we have already succeeded in demonstrating the distinction 

 between rations rich and poor respectively in lysine upon the growth 

 of these birds. Thus of two chickens from the same brood kept 

 upon diets strictly comparable except as to their protein constituents, 

 one upon a corn gluten food (low in lysine) gained only 52 grams in 

 55 days, whereas another on a corn gluten + lactalbumin food (rela- 

 tively rich in lysine) gained 283 grams in the same period, both having 

 been kept under precisely comparable cage conditions in the laboratory. 

 These results with birds, which agree so precisely with those obtained 

 with rats, practically demonstrate that the amino-acid requirements 

 shown by our experiments with the latter animal may be considered 

 as fundamentally essential to the nutrition of animals in general. 



Feeding trials with foods containing "protein-free milk" have 

 shown this to be greatly superior to artificial imitations containing 

 lactose and salts in the same proportion as in the natural product. 

 The known constituents of "protein-free milk" form upwards of 95 

 per cent of this material; consequently, among the remaining sub- 

 stances, we must seek for those which have such a potent influence 

 over the processes of maintenance and growth. Since our feeding 

 experiments have required the production of large quantities of 

 "protein-free milk," an exceptional opportunity has been afforded 

 for studying those constituents of milk which are present therein in 

 relatively small quantities. 



Our experience has long indicated that at least two such substances 

 are present; one, soluble in the butter-fat, is essential for long-con- 

 tinued growth; the other, soluble in water and associated with the 

 non-fat elements of the milk, is required for normal long-continued 

 maintenance. The experiments made by McCollum and his asso- 



