Donaldson, Brain and Spinal Cord of Rat. 165 



the difference becomes insigiiificaiit in comparison with the other 

 parts of the skeleton which contribute so much more to the total 

 result. 



We, therefore, conclude that a comparison between the body 

 length of the albino rat and the sitting height of man may be prop- 

 erly made. 



The purpose of making such a comparison is to determine whether 

 the rat is similar to man in the way in which this character changes 

 with age. 



It is not a character which at the time needs to be studied in 

 detail and so only very general statements are necessary. 



In his study on the growth of school children at Worcester, Mass., 

 West ('92) made records for the sitting height in both sexes be- 

 tween the ages of 5 years and 21 years. The results are charted 

 in his Fig. 1 (p. 32) and given in his Table 1 (p. 35). 



If we take the average values of the sitting height in man for 

 the two sexes, first at 19 years of age and again at 5 years of age, 

 we find the following: — 



Sitting height at 19 years 873 nmi. 



Sitting height at 5 years , 595 mm. 



Difference 278 mm. 



Percentage gain, 47 per cent. 



For comparison it is necessary to determine the increase in body 

 length in the albino rat during the corresponding interval. 



Computing from birth as the zero age, and taking the time unit 

 for the rat on one-thirtieth of that for man (see Donaldson, '06), we 

 obtain the following: — 



. Nineteen years of human age correspond with 220 days of rat 

 age. 



Five years of human age correspond with GO days of rat age. 



Table 9, in Donaldson, '08, shows that 220 days correspond with 

 an average body weight of 234 grams, and of 60 days, with 78 grams. 

 The corresponding body lengths in the rat, as shown in Table 2, 

 are for 



