122 CM. JACKSON 



200 grams. These weights, however, do not include the inter- 

 vertebral discs. 



The data for the dried cartilaginous skeleton (table 7 c) indi- 

 cate an even greater increase in the dry skeleton of the rats held 

 at constant body-weight. Thus in rats beginning at three 

 weeks the dry substance increases from 3.43 per cent of the body 

 weight to 4.98 per cent at six weeks of age, 5.49 per cent at eight 

 weeks, 5.84 per cent at ten weeks, 6.31 per cent at thirteen weeks 

 and 6.71 per cent at sixteen weeks. 



Since the increase in the dry skeleton is relatively greater than 

 that for the (moist) cartilaginous skeleton, it necessarily follows 

 that the skeleton must be losing in percentage of water and 

 gaining in percentage of dry substance. The percentage of 

 dry substance has been calculated for each individual skeleton 

 included in tables 7 a and 7 b, and the averages for each group 

 are as follows: controls at three weeks, 31.4 per cent; constant 

 three to six weeks of age, 33.5 per cent; three to eight weeks, 30.0 

 per cent; three to ten weeks, 41. 7- per cent; three to thirteen weeks, 



40.2 per cent; three to sixteen weeks, 44.0 per cent; control at 

 ten weeks, 53.4 per cent. 



Lowrey ('13) finds the dry substance of the ligamentous 

 skeleton m the normal albino rat to increase from an average of 



33.3 per cent at 20 days of age to 39.2 per cent at six weeks, 

 45.9 per cent at ten weeks, 50.4 per cent at five months and 52.6 

 per cent at one year. 



From the foregoing it is evident that in rats held at constant 

 body-weight beginning at three weeks, the growing cartilaginous 

 skeleton steadily increases its percentage of dry substance. 

 Thus it tends to change the proportions of water and dry sub- 

 stance as during normal growth. The percentage of dry sub- 

 stance does not increase so rapidly with age as during normal 

 growth, however, but lags behind corresponding to the retard- 

 ation in absolute growth. During inanition in adult rats, on 

 the contrary, there is a relative decrease in the dry substance, 

 and an increase in water-content (Jackson '15 c). 



It has already been noted in a previous section ("Lengths of 

 body and tail") that in the rats held at constant body-weight 



