214 CAROLINE M. HOLT 
But the chief reason for the difficulty in raising these rats was 
their lack of vitality. Although very active and playful, these 
animals were extremely frail little creatures, so weak that the 
slightest disturbance was likely to prove fatal. For example, 
an unusually active and promising test rat of fifty-three days, was 
carried from the colony to the laboratory for examination. As he 
appeared much excited, the carrier cage was set aside for an hour. 
The rat was heard running about for a time but at the end of the 
hour was found dead. The body weight of this rat was that 
of an animal fifteen days old and the brain weight was scarcely 
more. Young rats might appear lively and in every way normal 
in a late afternoon and be found dead in the cage next morning, 
for no reason to be discovered even after careful examination. 
Of nine such litters only two survived to the time of weaning, 
and these were kept with much difficulty. 
A litter of ‘runts’ was also included in this series. This was a 
litter of rats, all of which failed to grow normally, presumably 
because the mother had an insufficient supply of milk. They 
appeared in every way like the rats which had been stunted by 
underfeeding the mothers. 
b. Results. The general results of underfeeding in Series B 
were essentially the same in character as in Series A but they 
were considerably more marked (tables 9,10 and 11). The body 
length and general appearance of seventy-seven day rats, under- 
fed from birth, were practically the same as in normal three- 
weeks-old rats, save for the extreme cyanosed condition. 
From a comparison of Series Ai, As, and B, it becomes evident 
that it is easier to retard the growth of an eighteen day rat than 
of a rat thirty days old, and still easier to stop the growth of a 
rat at about the size of an eighteen day individual if the under- 
feeding is begun at birth. Moreover, it is obviously far more 
difficult to rear these animals underfed from birth than rats 
which have been allowed to get a good start of thirty days under 
favorable conditions and are therefore much more resistant to 
the deleterious effects of partial starvation. 
Effect on brain and olfactory bulbs. Series B shows brains 
actually lighter in weight at twenty-four to fifty-three days of 
