A Study of Growth 3 



there was difficulty in feeding certain animals. After this time 

 the average amount taken by each animal was 153 mg. in the 

 three feeding periods during the week. 



The weights of the salamanders were taken once a week, there 

 intervening a period of from forty-eight to fifty hours, on the 

 average, between the last feeding and the time of weighing; that 

 is, from Saturday until Tuesday. The animals were dried with 

 a cloth so as to get rid of as much of the moisture as possible, 

 and weighed in a beaker of known weight. The weights were 

 recorded in grams. 



Possible errors in the comparative weights deserve consideration, 

 for in all of the weights taken there were opportunities for positive 

 and negative error: (i) Error as a result of undigested matter 

 in the alimentary tract at the time of weighing. (2) Error from 

 not drying the animals uniformly at the same and at different 

 weighing periods. (3) Error from lack of uniform feeding, that 

 is, varying amounts of food. (4) Some individuals failed to eat: 

 this error, however, is lessened by the fact that where a certain 

 number of individuals failed to eat, the same number in the con- 

 trol for that set were intentionally not fed. (5) Finally, correc- 

 tions must be made for the death of one or more animals, which 

 may shift the average of the set. 



According to Minot,^ increase of weight or growth depends upon 

 two factors: (i) Upon the amount of body substance, the growing 

 material present at a given time; (2) upon the rapidity with which 

 that amount increases. The rate of growth may be expressed 

 as a fraction of the weight added during a given period. Minot 

 suggests taking the mathematical mean between the weights at 

 the beginning, and at the end of a period, in order to furnish a 

 basis from which to calculate the fraction, or the percentage 

 increment. The rate of growth may in this way be expressed 

 more accurately, for if during a period there is a definite incre- 

 ment, at the beginning of that period there is less material to 

 increase than at the end. New material is being added daily, 

 which in turn increases. By taking the mean, the actual amount 

 of growing material at a definite period is more accurately ex- 



' Senescence and Rejuvenescence. 



