BONE GROWTH. 65 



This decrease is not uniform but largely takes place in two stages 

 at which the reduction in growth capacity is markedly abrupt. 

 These occur at 30 and at 65 days of age. The latter brings the 

 growth capacity to the level which is approximately the same 

 for all bones and for both phases of growth, which is the level 

 maintained thereafter, and which is the level characteristic of 

 adult growth. 



The reduction at 30 days is attributable to the adjustment to 

 weaning, that at 65 days to the adjustment to puberty. These 

 phases of development have been discussed elsewhere ('250). 



There is no apparent systemic difference in degree of response 

 to weaning. With respect to the pubertal reduction, however, a 

 consistent difference is exhibited, in that the degree of cut in 

 growth capacity is greater for the femur than for the humerus in 

 both sexes. 



This systemic difference can be considered as a particular 

 expression of the general tendency of the growing organism to 

 reach that state of uniform level of growth capacity of all of its 

 parts, wherefrom those bodily proportions are maintained which 

 characterize the adult of the species. (The basis of this idea has 

 been sketched in an earlier publication ('230) : the full discussion 

 will be developed in a later analysis.) Assuming the correctness 

 of the hypothesis, it is obvious that since the growth capacity of 

 the femur during the preceding period of active differential 

 development is greater than that of the humerus, adequate 

 systemic approximation in growth capacity, which is brought 

 about in the case of these two serially homologous components of 

 the osseous system by the culmination of puberty, is only arrived 

 at through a differential systemic reduction in growth capacity 

 such as is shown here. 



The chart shows that in both sexes the growth capacity of the 

 femur in weight and length is more nearly like that of the body 

 in degree than is that of the humerus. In other words the 

 growth of the femur follows that of the body more closely than 

 does the growth of the humerus. The precise significance of this 

 systemic difference is obscure. It may be an expression of a 

 response to a need for supporting the greater part of the body 

 weight included in the posterior abdomen. In this connection it 



