When a diet of such character as would ordinarily meet all requirements is fed to a growing animal in amounts too small to meet the growth requirement, it is plain that such restriction may result in a deficiency of one, several, or all of the essential factors. If the diet is so selected as to be relatively rich in proteins, ash constituents, and the factors A and B, then restriction of the amount of food will result primarily in an energy deficit. Waters has described experiments which appear to have been of this character. He reports numerous cases of young cattle kept on restricted amounts of food of suitable kinds, the restriction being such as to materially retard the increase in weight as compared with that of a full fed animal of the same age, or even to hold the young animal at stationary weight at an age when it should have been growing rapidly. In such cases of insufficiency of the total food (energy) intake the skeleton continues to grow, in height at least, while adipose tissue steadily disappears, and the muscles become more or less depleted. In a young animal subjected to this type of undernourishment the skeleton grows in height to a much greater extent than in width. Thus in a full-fed steer the increase in length of foreleg and in width of chest were about equal, while in one whose rate of growth was retarded by sparse rations the width of chest increased only one third as much as the length of foreleg, and in another animal of the same age whose food was so restricted as to permit no increase in weight the increase of chest-width was only one eighth as much as the increase in foreleg. The ratios actually measured in typical cases were as follows:

Along with the narrower skeleton the underfeeding resulted in muscles of smaller diameter, absence of subcutaneous fat, and a general emaciated appearance. Young animals thus held at constant weight when they should be growing are in reality undergoing starvation. To quote from Waters' paper:

* Just enough food to maintain constant weight in an animal which should have been growing rapidly had he been more liberally fed.

"Apparently the animal organism is capable of drawing upon its reserve for the purpose of sustaining the growth process for a considerable time and to a considerable extent. Our experiments indicate that after the reserve is drawn upon to a certain extent to support growth, the process ceases and there is no further increase in height or in length of bone. From this point on, the animal's chief business seems to be to sustain life. This law applies to animals on a stationary live weight as well as those being fed so that the live weight is steadily declining, and indeed to those whose ration, while above maintenance, and causing a gain in live weight, is less than the normal growth rate of the individual. Such an animal will, while gaining in weight, get thinner, because it is drawing upon its reserve to supplement the ration in its effort to grow at a normal rate."

"On all the animals under observation the retardation in height growth did not manifest itself at all until after the sparse nourishment had been continued for several months. On the other hand, the influence upon the width development was observable much earlier, and width development ceased altogether, in the case of animals on a maintenance or submain-tenance ration, long before the height development had ceased."

"Our experiments have shown that within certain limits which are not yet at all well defined, retarded growth means retarded development of the organism. Thus an animal at twelve months of age and weighing on account of sparse nourishment only 400 pounds when it should under natural nourishment have weighed 800 pounds, has not its tissues as fully developed and matured as they would have been had the nourishment been normal. For example, we find that the flesh of steers 14-16 months old that had been sparsely fed throughout their lives presented the general characteristics such as color, flavor, etc. of veal or the flesh of calf. At this age the flesh of a highly nourished animal possessed the characteristic color, texture, and flavor of beef. Prof. Eckles has shown that Z dairy heifer calves heavily fed reach sexual maturity at from eight to ten months of age, whereas similarly bred individuals that were sparsely fed did not reach the stage of puberty under from 16 to 19 months of age."

"An animal which has been retarded and which in its earlier life has shown an asymmetric development, may tend later to correct this asymmetry, and it is not inconceivable that this may be fully corrected before the animal has reached a state of complete maturity, or a point where growth ceases altogether."

Somewhat similar experiments have been performed upon dogs by Aron. Here also when the food was suitable in character but too limited in amount to support normal growth the young animals grew in length and height but became thinner. Because of the "growth impulse" such an underfed young animal burns his reserve of body material to cover the deficit in the energy intake "in his endeavor to grow at a normal rate." Such a condition continued indefinitely results after a time in cessation of all growth and finally in death from starvation. A dog which by underfeeding had been kept for a year at the weight which he had when 5 weeks old and had become long, tall, and very thin, and was then fed liberally immediately gained in weight and circumference but appeared to have lost the capacity for further growth in length and height. If, however, the period of underfeeding be not too prolonged, the animal on subsequently receiving ample food may regain normal proportions and grow to full normal size.

Since stationary weight in the young animal which is attempting to grow with an insufficient energy supply does not mean cessation of all growth but growth of bone and brain at expense of adipose tissue and to some extent also of muscle, it follows that the body of such an animal gradually changes in composition, the percentages of fat and perhaps protein becoming less while the percentages of water and ash increase. If, however, the diet is rich in fat, as in experiments upon mice recently reported by Mendel and Judson, a simple diminution of the amount of food to a point where gain in weight ceases may not result in any such general replacement of fat by water, perhaps because in such a case the stunting may be due to insufficiency of some of the other factors rather than to an energy deficit.

The experiments of Mendel and Judson also yield interesting data regarding the changes which normally occur in the water, fat (ether extract), and ash content of the body during its most active growth. From 88 analyses of the entire bodies of mice the following changes in composition were found: (a) increase in solids from 16 per cent at birth to a maximum of 35 per cent at fifty days with a subsequent decrease to 33 per cent; (b) decrease in the proportion of water in the fat-free substance from 85.5 per cent at birth to 73 per cent in the adult mouse; (c) rapid increase in fat from 1.85 per cent at birth to about 10 per cent followed by slow increase to 12 per cent; (d) increase in ash content from 1.86 per cent at birth to 3 per cent in the adult mouse.