124 CM. JACKSON 



ble. In the skeleton of a rat held at constant body-weight 

 from three to ten weeks of age, most of the epiphyses of the 

 vertebral bodies have appeared, and they usually have united 

 at one end of each bone; the lower epiphysis of the humerus is 

 firmly united with the shaft, as normally at ten weeks; well- 

 developed third molars have appeared, as normally at ten weeks. 

 In a rat held at constant body-weight from age of three to six- 

 teen weeks, the skeletal differentiation was more advanced, 

 corresponding at least to the stage reached normally at ten 

 weeks, and in some respects perhaps even beyond it. 



These observations will suffice to establish the fact that the 

 skeletal growth during constant body-weight is accompanied 

 by normal developmental changes, as well as changes in chemical 

 composition (percentage of water). In other words, we find 

 not only increase in mass but growth and differentiation appar- 

 ently normal in character, though somewhat retarded in rate. 

 These skeletal characters therefore tend to correlation with age, 

 although influenced also by the general body-weight. 



The remarkable fact that the skeleton continues to grow while 

 the body-weight is held constant was apparently first observed 

 by Waters ('08) who found that calves previously well nourished 

 will continue to increase in height and in width of hip for a con- 

 siderable time, even when increase of bodj^-weight is prevented 

 by under-feeding. He remarks ('08 b, p. 9) : 



Apparently the animal organism is capable of drawing upon its 

 reserve for the purposes of sustaining the growth process, for a con- 

 siderable 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 to those being fed so that the live weight is stead- 

 ily declining, and indeed to those whose ration, while above main- 

 tenance, 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 supple- 

 ment the ration in its effort to grow at a normal rate. 



Aron ('11) experimented with dogs to determine the effect of 

 a restricted amount of food upon young, growing animals. He 



