132 CM. JACKSON 



SPINAL CORD 



When the relative weights are compared with the controls at 

 the beginning of the experiment (table 11), or with the theoreti- 

 cal normal according to Donaldson ('08) there appears a very 

 decided increase in the spinal cord at all the age-periods during 

 the experiment. Thus while the body-weight has been held 

 constant from the age of three to that of ten weeks, the spinal 

 cord has apparently increased from an average of 0.179 to 0.243 

 grams, an increase of about 36 per cent (or slightly more if the 

 initial weight be decreased to correct for the difference of body- 

 weight at three weeks, 24.5 grams, and ten weeks, 24.0 grams). 

 This corresponds to an increase from 0.74 per cent to 1.02 per 

 cent of the net body- weight. The increases at the other age- 

 periods are equally striking. 



Donaldson ('11) in the experiments previously mentioned also 

 found an increase in the weight of the spinal cord in rats held at 

 body-weight of about 34 grams from the age of thirty days to 

 that of fifty-one days. He does not estimate this increase exactly 

 but from the normal weight of the cord at the beginning of the 

 experiment (cf. Donaldson '08, table 1) the weight must have 

 increased from about 0.223 to 0.2498 grams, an increase of about 

 10.7 per cent. While this is not so striking as my results fper- 

 haps in part because my experiments covered a longer period of 

 time) it agrees in indicating 'during constant body-weight a 

 much stronger growth tendency in the spinal cord than in the 

 brain. This is in agreement with the well-known fact that in 

 general the normal post-natal growth of the spinal cord is rela- 

 tively much more rapid than that of the brain. This growth of 

 the spinal cord is apparently correlated with the increase in 

 trunk-length (Donaldson) . 



EYEBALLS 



An increase even more striking than that of the spinal cord is 

 apparent in the eyeballs (table 12). In rats held at constant 

 body-weight from the age of three weeks, the eyeballs increase 

 from a relative weight of about 0.50 per cent of the body-weight 

 to 0.64 per cent at six weeks, 0.82 per cent at eight weeks, and 



