WEIGHTS OF UNDERFED YOUNG ALBINO RATS 307 
for various periods. The total number of hours that the test 
rats of a litter were separated from the mother during the first 
three weeks (504 hours) after birth averaged 260 hours (213 to 
285 hours) or more than half of the total period. Briining (’14) 
was able to isolate young rats from the mother for only about 
one-fourth of the total nursing period. 
In most instances when young fasting rats were returned to 
the nest, the mother would immediately take care of them. 
Occasionally, however, after being disturbed, the mother would 
abandon her litter entirely. In these cases it was often possible 
to save the young rats by putting them to nurse with other 
mothers whose young were fasting at that time. By continually 
putting young rats of different litters in the nest, it was found 
possible to keep one female nursing for twelve weeks. 
The age of weaning for the albino rat is usually given as three 
weeks. However, Donaldson (15) (‘The Rat,’ p. 19) states 
that the young, if permitted, will continue to depend partly 
on the mother for some days longer. One of my litters was 
observed to continue nursing until six weeks of age. 
When three weeks old, the test rats underfed from birth, and 
_ also those used in the experiments starting at three weeks, were 
placed on the bread and milk diet, receiving only a limited and 
carefully measured quantity daily. As was observed by Jackson 
(15 b) and Stewart (16), the young rats in the present experi- 
ments were kept at maintenance fora considerable period upon 
a gradually diminishing ration. Thus for one litter (St 96) dur- 
ing 30 days underfeeding the daily amount of food required for 
maintenance of body weight at approximately 14.5 grams de- 
creased from an average of 3.87 grams to 2.97 grams per rat. 
Later, however, the maintenance ration apparently becomes 
more nearly constant. For example, in the case of another litter 
(St 46) underfed for a very long period the total daily weight of 
food consumed by 5 rats from the 131st to the 180th day of the 
experiment was constantly 33 grams. The average body weight 
of these rats during this period remained unchanged (47.6 grams.) 
Stewart (’16) for one litter found no decrease in the maintenance 
ration from the 60th to the 120th day of the experiment. 
