74 



GROWTH IN PARTS 



fully dissected off and the skeleton, including bones, cartilages and liga- 

 ments, was weighed. This weight, subtracted from that of the skele- 

 ton and musculature together, gives the weight of tlie musculature, in- 

 cluding the tendons. Evaporation was reduced to a minimum by 

 keeping the various structures in a closed moist container, so far as 

 possible. The net body weight, which is the gross body weight minus con- 

 tents of stomach, intestines and urinary bladder, was used as the basis in 

 calculating the percentage weights. The percentages therefore differ 

 slightly from those calculated upon the gross body weight. The dif- 

 ference is not of material importance in the case of the albino rat, 

 however, as the intestinal and other contents do not average more than 5 

 per cent of the body at the ages observed (excepting at 6 weeks, where 

 the average was about 8 per cent.) The observations were grouped at 

 seven ages, chosen for the following reasons. At one week the weight 

 at birth has about doubled. At three weeks it has about doubled 

 again, and this moreover is the age at which the animal is usually 

 weaned. At six weeks the body weight has again about doubled, and 

 the animal is well established upon its permanent diet. Ten weeks 

 represents the age of puberty, and the body weight of six weeks has 

 again about doubled. At one year the body weight has again nearly 

 doubled, and this represents nearly the adult weight. Five months 

 was arbitrarily selected as the time when the body weight is approxi- 

 mately half way between those of ten weeks and one year. While 

 therefore observations are not available for the various intermediate 

 age periods, these are sufficiently close together so that no important 

 change in the relative weights of the constituent parts is likely to be 

 overlooked. Moreover, on account of the variations at the different 

 ages in the body weights, these form a fairly continuous series ; and the 

 relative weights of the various constituent parts are apparently more 

 closely correlated with the body weight than with the age. 



The relative weights of the component parts examined are 

 given in table 49 (modified from table 2, p. 455, loc. cit.). 



TABLE 49 



Albino rat Average percentage weight of head, trunk and extremities at various 

 ages sexes combined (Jackson and Lowrey, '12) 



