WEIGHTS OF ORGANS IN UNDERFED YOUNG RATS 121 



is held constant the skeleton continues to increase in weight to a 

 marked degree. In the rats beginning at three weeks, there is 

 an increase in the relative weight of the ligamentous skeleton 

 from 15.7 per cent of the body to 18.0 per cent at six weeks and 

 to an apparent maximum of 23.7 per cent at eight weeks. This 

 latter is probably an exceptional figure, as in the largest group, 

 at ten weeks, the average is 21.2 per cent (fig. 4). This corre- 

 sponds to an increase from an absolute weight of 3.90 to 4.98 

 grams, an increase of about 28 per cent (or slightly more, if 

 correction be made for the difference in body-weight, average 

 24.5 grams at three weeks and 23.8 grams at ten weeks). The 

 two cases carried to thirteen and sixteen weeks, respectively, 

 show a slightly smaller relative increase. The rats used at later 

 and longer periods (ages of six to thirty-two weeks and ten to 

 thirty-five weeks) also show a considerable increase in the 

 skeleton, though relatively less than those beginning at the 

 earlier period. 



The data for the cartilaginous skeleton (table 7 b) similarly 

 show a marked increase in rats held at constant body-weight for 

 various periods beginning at three weeks of age. The figures 

 for the largest group (three to ten weeks) indicate an increase 

 from 11.4 per cent to 14.6 per cent of the body. In terms of 

 absolute weight, the increase is from an average of 2.60 grams 

 (body- weight 22.9 grams) to 3.16 grams (body-weight 22.4 grams), 

 an increase of about one-fourth. Subtracting the percentage 

 weights of the cartilaginous skeleton from the corresponding 

 ligamentous skeleton, there is (for the three to ten weeks group) 

 an evident increase of the ligaments and periosteum from 4.3 

 per cent to 6.6 per cent of the net body-weight. This would 

 indicate that the ligamentous component of the skeleton shares 

 in the marked growth during constant body-weight. 



Professor Donaldson (in a personal communication) has kindly 

 supplied a series of observations showing that the cartilaginous 

 skeleton in the normal rat changes from a relative weight of 

 about 10 per cent of the body at 20 grams to 7.5 per cent at 50 

 grams, 7 per cent at 100 grams and 6.7 per cent in rats above 



