WEIGHTS OF UNDERFED YOUNG ALBINO RATS 347 
Thus the liver, ovaries, kidneys, heart and spleen show a marked 
tendency to increase in weight in the rats underfed from birth to 
ten weeks, but not in those underfed from three to ten weeks. 
On the other hand, in my rats fasting for very long periods, the 
hypophysis (in the males), alimentary canal, testes and epididymi 
show a marked loss in weight, which does not occur in rats under- 
fed for shorter periods. 
The present experiments thus emphasize the fact that with 
advancing age, many of the different organs show a changing 
tendency in their growth reaction during underfeeding. The 
same organ (for example, the heart or alimentary canal) in young 
animals during the earlier periods of fasting may manifest a 
marked tendency toward continued growth while the increase 
in body weight is greatly retarded, but later may barely maintain 
itself, and finally perhaps even lose weight. Other organs, like 
the eyeballs, spinal cord and skeleton, continue to grow or main- 
tain their weight with remarkable persistency; while still others 
(thymus) show a marked loss throughout all periods of inanition. 
These changes in weight in the various systems and organs. of the 
young body during inanition are probably correlated with histo- 
logical, chemical and physiological changes concerning which as 
yet but little is known. 
SUMMARY 
The more important results of the present investigation may be 
summarized briefly as follows: 
Newborn albino rats are able to withstand separation from the 
mother for nearly one-half of the total time for the usual nursing 
period of three weeks, resulting in great retardation of the normal 
growth of the body as a whole. 
During the underfeeding, the tail in the test rats elongates more 
rapidly than the body, thus producing relatively long-tailed 
individuals. 
The eyelids in the test rats open at a body weight lower than in 
the controls, but the time of opening is somewhat delayed. 
As to the body proportions, the weights of the extremities are 
