240 HENRY II. DONALDSON 



be explained later. The two sexes have been treated together, 

 since there is no clear sex difference in the relative weight of the 

 skeleton. All weights are given in grams and all lengths in 

 millimeters, and it must be repeated that all the values recorded 

 are those obtained after treatment with the 'Gold Dust,' which 

 reduces the fresh weights of the bones and especially those from 

 rats less than 100 grams in body weight or about 70 days in age, 

 as shown in table 34, Appendix 2. 



METHOD OF WEIGHING THE BONES 



The bones were weighed in closed bottles on balances sensitive 

 to 0.0001 gram. This procedure is simple and not subject to 

 serious observational errors except for the weighing of the 'moist 

 or fresh bones,' i.e., the bones immediately after maceration and 

 before they are allowed to dry at all. To obtain this fresh value 

 for the bones the effort has been made to remove, by wiping on 

 filter-paper, all evident fluid adhering to the surface. This is 

 readily accomplished in most cases, but the moist cranium 

 requires special preparation before it is thus weighed. 



In this case the external surfaces can be readily wiped, but it 

 is also necessary to remove with strips of filter-paper any accu- 

 mulated fluid from the nares, the bullae, and the cavity of the 

 cranium. With due attention, however, to this preliminary 

 preparation of the cranium, satisfactory weight data may be 

 obtained. The moment one has to deal with dry bones, the 

 difficulties just mentioned disappear. 



ARRANGEMENT OF DATA IX THE TABLES 



As the weight of the skeleton was the value to be found, the 

 106 rats in this series were arranged according to body weight. 

 It is well known, however, that the observed body weight of the 

 rat fluctuates readily, and especially in old animals, tends to fall 

 off. In order to make the entries systematic, a fixed procedure 

 was followed for the establishment of the normal body weight. 

 The body length of each rat was taken, and, as this does not 

 fluctuate with the nutritional condition of the animal, it was 

 used as the standard. 



