222 ADVENTURES IN RADIOISOTOPE RESEARCH 



injecting the rabbit repeatedly with labelled phosphate. After the laps 

 of 50 days, the phosphorus of the femur epiphysis found to have a spe- 

 cific activity of 30% of that of the plasma inorganic P, thus indicating 

 that 30%, and only 30%, of the epiphysial bone apatite had been rene- 

 wed, a much lower renewal figure (7%) being obtained for the diaph3^sial 

 phosphorus. 



This method has the disadvantage of being cumbersome. Furthermore, 

 the results may be influenced by the time that passes between the last 

 injection of the rabbit and the killing of the animal. Therefore, when 

 determining the renewable fraction of the skeleton calcium of the mouse, 

 we have chosen another procedure. Mice were bred whose skeleton was 

 labelled throughout with ^^Ca and the loss of the activity in the skeleton 

 was followed with increasing age of the animals. Such mice can be 

 obtained by administering to the mother food containing labelled cal- 

 cium already weeks before gestation and continuing to feed the lactating 

 mother and the growing offspring with food containing labelled calcium. 

 We assume every one of the offspring to have the same '*^Ca content. 

 If we stop administering labelled food after these offspring are out- 

 grown, they start to interchange their labelled bone calcium with the 

 unlabelled calcium from the food with the result that the *^Ca content 

 of the skeleton decreases and, when the offspring is killed after two 

 months, its *^Ca content is lower than that of another offspring killed 

 after one month. By killing members of a litter at different dates, we 

 can follow the processes in the skeleton for years, viz. through the 

 lifetime of the animal. 



Table 1. — Weight and Activity 

 OF New-born Mice 



The •*^Ca content of every member of a litter is not strictly the same, 

 and this applies also to the growth rate. The evidencee that a part of the 

 curve depicted in Fig. 1 (p. 224) is discontinuous may presumably be due to 

 a difference in the uptake of maternal '^^Ca by the offspring of the same 

 litter. The variation in the radioactivity of different members of a litter, 

 however, is restricted and does not suffice to frustrate the applicability 

 of the method described (cf. Table 1). 



