POSTNATAL GROWTH IN THE ALBINO RAT 5 



Since the number of observations was somewhat limited, it was 

 thought better, especially for the study of variation with age, to 

 restrict them to certain definite ages, rather than to scatter them 

 over the entire period. Seven ages were chosen for this purpose, 

 namely, newborn (one day or less), seven days, twenty days, six 

 weeks, ten weeks, five months, and one year. These ages were 

 selected for the following reasons. At seven days, the body 

 weight at birth has about doubled. At twenty days, the weight 

 has approximately doubled again, and at this age the albino is 

 usually weaned. At six weeks, the body weight has somewhat 

 more than doubled again, and the animal is well established upon 

 the new diet. At ten weeks, the rat has again about doubled 

 its weight, and sexual maturity is reached. At one year, the 

 weight has again approximately doubled, and at this age, accord- 

 ing to the observations of Slonaker ('12), the albino rat (in sta- 

 tionary cages) has nearly reached its maximum body weight. 

 Five months was arbitrarily selected as a point intermediate 

 between ten weeks and one year. 



A few observations upon the body weight at fourteen days and 

 at thirty days are also recorded -in table 1. The continuous 

 growth curves for the absolute weight of the various organs, in 

 terms of body weight, are given by Hatai ('13). It is hoped that 

 the age periods selected for the present paper are sufficiently close 

 together so that no material change in variability, correlation, 

 and so forth, will be overlooked. 



In some cases, as previously mentioned, only the gross body 

 weight was observed. This weight was always taken in the fore- 

 noon, before feeding. In the cases where the animal was to be 

 dissected, it was killed by chloroform, and the gross body weight, 

 and lengths of trunk and tail recorded. The head was then 

 removed on a plane just posterior to the cranium and anterior 

 to the larynx, and was weighed; while the trunk was suspended 

 by the tail, allowing the blood (unmeasured) to escape. The 

 eyeballs and in some cases the brain were then removed and 

 placed in a closed jar upon moist filter paper. Next the trunk 

 was dissected and the following organs successively removed: 

 thyroid gland; thymus (dissected out of surrounding fat); heart 



