318 E. R. HOSKINS 



the net body weight and in the younger males about 6.1 per cent. 

 These correspond fairly well with the estimate of Jackson ('15) 

 which was 7 per cent of the adult net body weight, and with Con- 

 row's observations (cited by Donaldson '15, table 53). The dif- 

 ference between the sexes is accounted for by the heavier body 

 weight (with correspondingly lighter skeleton) in the male 

 groups, rather than by any true sexual difference. 



The older groups of each sex on the average appear to have 

 a relatively heavier skeleton than the younger. This is con- 

 trary to the general tendency of the skeleton during growth to 

 lag behind in relative weight. In these groups, however, the 

 differences in body weight are much less than usual for the cor- 

 responding age differences, and the increased weight in the 

 older skeletons is possibly due to more advanced stages of ossi- 

 fication and calcification. Thus in two animals of the same 

 body weight, the older apparently has a heavier skeleton. This 

 tendency is not evident in Conrow's data (cited by Donald- 

 son '15), however. 



In this connection may be cited the observations of Jackson 

 ('15) who found that in young rats held at constant body weight 

 by underfeeding the skeleton continues its development (dif- 

 ferentiation and increase in wet and dry weights). It is there- 

 fore probable that the relative weight of the skeleton depends 

 somewhat upon the age factor, as skeletal growth is to some 

 extent independent of the general growth of the body. 



The dry cartilaginous skeleton is likewise relatively slightly 

 heavier in the female (3.7 to 4.6 per cent) than in the male (3.2 

 to 3.8 per cent), and the older rats of both sexes have relatively 

 heavier dry skeletons than the younger. The explanation for 

 this is doubtless the same as that above given for similar rela- 

 tions in the weights of the wet skeleton in different groups. 

 There is considerable variation shown by individuals, so that 

 the average values for both wet and dry skeleton can be con- 

 sidered as only approximate. Differences in the technique 

 may also modify the skeletal weight considerably. For example, 

 Conrow's data for the dry skeleton (cited in Donaldson's table 

 53) are too high, because her method of drying at room tempera- 



