WEIGHTS OF UNDERFED YOUNG ALBINO RATS 325 
from an average of 33.3 per cent at twenty days of age to 52.6 
per cent at one year. 
Further evidence of growth continuing according to the normal 
process is indicated by the appearance of the third molar in the 
mandible and maxilla of the test rats underfed from birth to ten 
weeks of age. This tooth is not visible in the Jaws of the younger 
controls with the same body weight. 
The various individual bones of the skeleton in the underfed 
young rats are not only larger but also more advanced in their 
stage of development. The epiphyses at the ends of the humerus, 
especially at the distal end, are well fused with the shaft, much 
more so than in the case of the younger controls. Also the 
epiphyses at the ends of the femur and tibia are more completely 
developed in the test animals at ten weeks of age than in the 
controls, and in the former only has the tibia fused with the dis- 
tal end of the fibula. 
In general, therefore, the results confirm the observations of 
Jackson (15 b) that during maintenance of body weight in 
young animals, not only skeletal increase in mass occurs, but 
also skeletal growth and differentiation of apparently normal 
character, though somewhat retarded in rate. 
Waters (’08), who found that calves continue to increase in 
height and width of hip for a considerable time when held at 
maintenance, was probably the first to observe the fact that the 
skeleton continues to grow in young animals even when the body 
weight is held constant. 
Aron (’11) noted an increase in the length and height of young 
dogs held at constant weight for considerable periods. He also 
found an increase in nine of the individual bones, but made no 
observations upon the entire skeleton. The first complete and 
systematic study of the growth of the skeleton in young animals 
held at maintenance was by Jackson (15 b). 
Birk (11), Aron (14), Hess (716) and others show that a strong 
growth tendency of the skeleton is apparently manifested also 
in children during malnutrition. 
During acute and chronic inanition in adult rats the skeleton 
nearly maintains its original weight (Jackson 715 a). 
