PHYSIOLOGY: MENDEL AND JUDSON 
693 
conclusions as to the development of the skeleton during arrested growth 
which are based on an increased ash content of the body, should depend 
upon comparison not with the ^normal animal/ but with the normal 
animal on the same diet. 
When abundance of fat is furnished in the diet, but not enough pro- 
tein to maintain normal growth, the fat content of the animal is greater 
than when the food contains an adequate amount of protein with the 
same proportion of fat. There seems to be a tendency to protect the 
limited amount of protein by increasing the available supply of fat in 
the body. This does not occur when growth is arrested by lack of ly- 
sine, as in the use of gliadin as the only protein in the diet, since in this 
case the limiting factor lies not in the amount but in the nature of the 
protein. 
By underfeeding, mice have been completely arrested in growth, as 
far as growth is expressed in gain in weight, and have been maintained 
at a constant body weight of 12 grams for thirty and sixty days. At 
the end of the thirty-day period, control mice of the same initial weight 
as the experimental animals, and on the same diet, weigh 22 grams. 
Comparison of the composition of the stunted mice with that of mice 
growing normally shows that the proportion of fat in the stunted animal 
is about the same as in the normal mouse of the same weight (but young- 
er), while the percentage of water in the fat-free substance corresponds 
to the water content of a normal mouse of the same age (but heavier). 
That there is no evidence of a general replacement of fat by water, 
such as is often reported in underfed animals, may be due to the large 
proportion of fat used in this diet. The ash content, both absolute 
and relative, of the stunted mouse is greater than that of the normal 
mouse of the same weight, confirming in this the results reported by 
Aron for the rat, and indicating continued growth of the skeleton 
under conditions which prevent the animal from gaining in weight. 
The tendency of the skeleton to develop under such adverse condi- 
tions does not appear to be as strong in the mouse, however, as in the 
rat, if the few data reported for the rat are representative. 
Retardation of growth by other means — reduction of protein or of 
salts in the food, or substitution of gliadin for other proteins — affects 
the ash content of the animal in the same way as simple underfeeding. 
If young mice which have been maintained for a time at constant 
weight are given sufficient food again, they grow at a greatly acceler- 
ated rate which enables them to overtake control mice which have 
grown uninterruptedly. The ash content, however, does not increase 
at the same rate as the body weight; and the development of the skele- 
