24 o SCIENCE PROGRESS 



It will be observed that the set of six rats fed on this diet 

 grew slowly until the thirteenth day, when growth ceased. By 

 the twentieth day loss of weight had set in, and a week later 

 five of the six rats had died. Compare now the record of the 

 six rats fed on the same diet with the addition of 2 c.c. of milk 

 per rat per diem, represented by the line of blackened circles. 

 A steady growth in size and weight has occurred— a growth 

 resulting in the doubling of the weight of the animals in less 

 than twenty days— in other words, a thoroughly normal 

 growth. 



In the next figure is contrasted the effect produced by feeding 

 eight rats on the purified artificial diet with another eight rats 

 fed on the same diet plus 3 c.c. of milk per rat per diem. On 

 the eighteenth day the milk ration was transferred from the 

 one set to the other. The arrest of growth of the first set and 

 the setting in of the delayed growth of the second set are most 

 apparent effects of the change. 



In all the experiments, of which the above are only typical 

 examples, it must be realised that the amount of solid matter 

 in the added milk was almost negligible in comparison with 

 the weight of artificial food taken, and amounted usually to 

 from 1 to 4 per cent, of the total food. Moreover, by far the 

 larger part of this additional material consisted of lactose or 

 protein, both of which were shown by separate experiments to 

 be totally devoid of growth-activating properties. 



Further experiments of a similar nature have shown that 

 the same accelerating effect on growth may be obtained by 

 adding to the artificial diet traces of the protein-free alcoholic 

 extract of the milk solids. A still further refinement has been 

 possible in that it has been found that the ethereal extract of 

 the alcoholic extract of milk solids is just as efficient as the raw 

 milk. Evidently the responsible agent is not inorganic in 

 nature, and it must also be very powerful in its action. The 

 substance is thermostable, because boiled milk is just as effective 

 as unboiled milk. 



The experiments of Osborne and Mendel have confirmed 

 the work of Hopkins, and have extended it in several directions. 

 They also have found that rats cannot be maintained on an 

 artificial diet containing amounts of protein, fat, carbohydrates, 

 and salts which are more than sufficient to satisfy the energy 

 requirements of the animals. If, however, dried protein-free 



