106 GROWTH 



out on rats. In the first the underfeeding period was initiated at 

 birth, by separating the young from the mother at various in- 

 tervals. This period of underfeeding varied from three to about 

 ten weeks. At the end of the ten-week period the underfed rats 

 weighed about 16 grams, while the normal weight at that age 

 is over iOO grams. Of the twenty-three rats underfed for three 

 weeks, five equaled or exceeded their normal controls at ma- 

 turity. The average, however, was distinctly lower. Of the nine- 

 teen rats underfed to about ten weeks, one female attained ulti- 

 mately a weight equal to that of the controls. The others, how- 

 ever, were under the weights of the control animals. 



The second series was placed on the deficient rations at an age 

 of three weeks, and the period of underfeeding lasted for 

 seventeen weeks in one group, and until the animals were nearly 

 one year old in the other. In this latter group the average weight 

 was about 60 grams when refeeding began. In all cases, both 

 groups, the result was permanent dwarfing of the underfed 

 rats. When all cases of underfeeding are considered, Jackson 

 and Stewart estimate roughly that the dwarfing varied from 

 about io per cent in the milder cases to about 35 per cent in the 

 most severe. 



It is clearly evident that the length of the underfeeding pe- 

 riod, and the age at which underfeeding begins, are important 

 factors in determining the extent of the injury. Aron 5 ' had 

 noted that if rats were underfed from 50 to 1 50 days they were 

 permanently dwarfed. Stewart 58 obtained complete recovery 

 from underfeeding that began at three and extended to ten 

 weeks of age. In the series of Jackson and Stewart, the effect 

 of age is clearly demonstrated. If the underfeeding begins at 

 birth, a period of three to ten weeks causes permanent dwarfing, 

 but if it begins at three weeks of age a period of similar length 

 leaves no permanent effects. 



In view of the observations of Aron, of Jackson and Stewart, 

 and of others, some earlier observations acquire added signifi- 

 cance. Thus Dunn 59 and King 60 found that animals small at 



