68 FRKDKRICK S. HAMMETT. 



for the growth capacity of the bones of the male in weight and 

 length to be more typical of that of the body than is that of the 

 bones of the female during the period of active differential 

 development. 



The course of development of the bones in weight is sex- 

 specifically different from that of the body. This major differ- 

 ence occurs at the same time in each sex (e.g., during the growth 

 period from 30 to 50 days of age). In the male the growth 

 capacity of the bones increases, while that of the body remains 

 practically constant: in the female the growth capacity of the 

 bones remains practically constant, while that of the body is 

 markedly decreased. An explanation of these differences is not 

 at hand. It is sufficient for the present purpose to point out 

 that the determination of the course of growth of these bones in 

 relation to that of the body is factored by sex. 



From the foregoing it is evident that systemic correlation is 

 closer than sex correlation, in both rate and type of development, 

 particularly during the period of active differentiation. This 

 indicates that the sex-specific factors of influence are super- 

 imposed upon the systemic specific determinants of bone growth. 

 These sex-specific factors are probably as much if not more 

 attributable to immediate incretory influences emanating from 

 the gonads, than to the more remote and generalized determinants 

 of genetic origin. This belief is sustained by the fact that the 

 occurrence of these differences is correlatable in time of appear- 

 ance with the exhibition of certain sex-specific stages in gonadal 

 development. This has been discussed in detail in the report 

 on the chemical differentiation of the bones ('250). 



Certain other derivatives are of biological interest. 



As a result of sex differences in the relative rates of growth, 



the length of the bones tends to be greater in proportion to the 



length in the female than in the male, at all ages. This 



indicates that the appendicular skeleton of the female when 



compared with the axial is longer than that of the male. 



A similar superiority of the humerus and femur of the female 



in weight with respect to body weight is exhibited from 65 days 



It is possible that the skeletal system of the sexually 



)ino rat is a greater proportion of the body as a whole 



in the female than in the male. 



