PATH OF ATOMS THKOKiH GENERATIONS 237 



1 milligram of the phosphato of the bone skeleton shows tlie same radio- 

 activity as 1 milligram oi' phosphate of the blood fluid, it could ))e 

 concluded thai the total phosphate content of the bone skeleton was 

 renewed during the experiment. This, however, is not all the case. The 

 experimental conditions for determining the part of the bone skeleton 

 which is renewed are very unfavourable. The specific activity of the 

 plasma phosphate falls at first very rapidly, later on very slowly, and 

 shows fluctuations during the necessarily long duration of the experiment . 

 In order to simplify the conditions for the experiment, the specific activity 

 (activity per milligram) of the bound phosphate of a rabbit was held 

 constant for 50 days by repeated daily injections. 



It followed that about one-third of the soft epiphysial bone skeleton 

 was renewed during the duration of the experiment (11). From the 

 hard diaphysial bone structure, a considerably smaller fraction is re- 

 newed. The exchangeable part of the bone structure of the rabbit has 

 therefore to be limited to about one-third. 



FATE OF CALCIUM ATOMS 



In order to be able to pursue the fate of calcium through several 

 generations of mice, we had to know what fraction of the calcium ion 

 in the bone skeleton of mice is renewed during a lifetime. Numerous 

 experiments concerning the calcium metabolism of the skeleton have 

 l)een described (12—16). In determining the fraction of the exchange- 

 able skeleton calcium, we continued the afore-mentioned experiments, 

 and we replaced the very tedious method by another. We did not investi- 

 gate the replacement of the inactive bone calcium by labelled calcium 

 in the fluid blood. We bred animals that were evenly marked with 

 radioactive calcium, calcium-45. We determined then the fraction of 

 radioactive calcium ions that left the animal during its lifetime and that 

 were replaced by inactive calcium ions from the food. We supplied 

 the mother with radioactive calcium of limited activity (less than 0.5 

 microcurie) so that no radiation damage to the animal had to be feared. 

 In this way we obtained an evenly activated generation of mice, which 

 cannot be obtained by any other method. Each member of a single 

 generation had the same calcium-45 content within a few percent. 

 The mice received radioactive calcium with their food till they were 

 grown up. One of the siblings was killed at birth, and the radioactivity 

 of its skeleton was determined. The remaining animals were killed at 

 other times and examined. It became evident that after more than 

 1 year, which repre-sents a considerable part of the life span of a mouse. 

 6.7 ± 7.9 percent of the original calcium atoms of the skeleton could 

 still be found. Therefore onlv one-third of the bone skeleton is renewed. 



