GROWTH OF THE SKELETON 293 



SUMMARY OF THE OBSERVATIONS ON THE RAT 



The entire cartilaginous skeleton increases in absolute weight 

 rather steadily from birth to the end of the record (chart 1, 

 table 2). 



In relative weight it increases up to about 15 grains of body 

 weight, and after that decreases. With the method of macera- 

 tion here used, the cartilaginous skeleton represents in the 

 mature rat about 5 per cent of the total body weight (chart 2, 

 table 2). 



On considering the two main components of the skeleton — the 

 axial skeleton and the appendicular — it appears that the ap- 

 pendicular skeleton is always smaller than the axial, but that 

 after a body weight of 15 grams, the relative weights of the two 

 parts become nearly constant, though the appendicular division 

 grows a trifle more slowly (chart 2, table 2). 



When studied from the standpoint of relative weight, it is 

 seen that the early rise in relative weight is more marked in the 

 appendicular skeleton, indicating a rapid increase in the weight 

 of the limb bones just after birth (chart 2, table 2). 



The cranium shows a steady increase in absolute weight, but 

 the rate of growth is only about two-fifths of that for the entire 

 skeleton, and the cranium therefore diminishes in its relative 

 weight more rapidly than the rest of the skeleton as the rat 

 increases in size (chart 3, table 3). 



If the two divisions of the appendicular skeleton are examined 

 separately, it appears that the most marked growth before a body 

 weight of 15 grams occurs in the pelvic girdle and appendages, 

 indicating the relative immaturity of the hind limbs at birth, but 

 the two girdles come into their mature relations at puberty 

 (body weight, 100 grams) (charts 4 and 5, table 4). 



When the data are plotted so as to give the growth of the 

 skeleton -on age, the general form of the graphs is sinuous, like 

 the graphs for the growth of the body as a whole, and in addition 

 the graphs for the two sexes diverge at about sixty days, when the 

 differences in body weight according to sex become clearly marked 

 (chart 6, table 5). 



