WEIGHTS OF ORGANS IN UNDERFED YOUNG RATS 111 



The two observations on rats held constant from the age of 

 three weeks to the ages of thhteen and sixteen weeks would seem 

 to indicate a continuation of the process of elongation of the tail, 

 even beyond the normal ratio at corresponding age, but the 

 number of observations is too small for definite conclusion. 



The number of animals beginning the experiment at later 

 ages, body-weight constant from age of six to age of thirty-two 

 weeks (2) and from ten to thirty-five weeks (3) , is also too small 

 to draw any very positive conclusions. However, so far as they 

 go, they indicate (table 3 b; fig. 2) that between the ages of six and 

 thirty-five weeks there is no marked change in the tail-ratio of rats 

 held at constant hody-iceight. 



It will be observed that also in the normal rats between six 

 and thirty-five weeks of age there is no apparent change in the 

 tail-ratio, whereas between three and six weeks of age there is 

 normally a decided increase in the tail-ratio. The lengths of 

 the body and tail are of course determined primarily by the 

 growth of the skeleton. I would therefore interpret the results 

 concerning the lengths of tail and body as follows. In young, 

 growing rats held at constant body-weight, the body and tail 

 tend to increase so as to assume the normal ratio at corresponding 

 ages. This is due to the fact that, as will appear later, the skele- 

 ton continues to grow in a normal manner (though at a reduced 

 rate) in animals held at constant body-weight. 



There is another possible factor in causing the increased tail- 

 ratio in young, growing rats, which may also apply to the similar 

 relative elongation of the tail found in adult rats (cf. Jackson 

 '15 c). Professor Donaldson points out (in a personal com- 

 munication) that during inanition there may be an arching of 

 the spinal column, producing an actual shortening of the body- 

 length. Such an arching actually does occur, and may be noted 

 especially in young rats durmg chronic inanition. It is well 

 shown in the stunted rats photographed in figure 1. Of course 

 the greater part of this longitudinal curvature of the spinal column 

 is eliminated by the slight tension exerted in order to straighten 

 out the body when it is measured after death. But it is still 



