238 



ADVENTURES IN RADIOISOTOPE RESEARCH 



In another series of experiments we examined newly born mice right 

 after birth. The other mice were transferred to an inactive foster-mother 

 and analyzed at different times in the following 1.5 years. Figure 2 

 shows the results of some of these experiments. One should expect that 

 during the intense growth of the first week of life a large part of the atoms 

 taken over from the mother are disposed of and replaced by those from 



I 



100 



80- 



^ 60 



c 



<u 



■*— 



c 

 o 

 o 



o 40 

 o 



20- 



100 



200 



300 

 doys 



400 



500 



600 



Fig. 2. Loss of inlierited calcium atoms during the life of a mouse. 

 We determined the total activity of sibUngs whose mother was fed 

 calcium-45. The offspring were killed at different times 



food. In the first 50 days, the loss of atoms from the mother is consi- 

 derable. However, in the whole life of a mouse it does not amount to 

 more than 50 percent. 



We found about 1/300 of the labelled calcium atoms that the mother 

 of the second generation received at its own birth, in the newly born 

 of the third generation. The loss in calcium atoms of the ances- 

 tors before propagation and through limited transmission to the off- 

 spring is repeated from generation to generation. It can therefore 

 be found by extrapolation that, of the 6 X 10^^ calcium atoms to be 

 found in a mouse weighing 30 grams, not one is present in the 

 11th generation of its descendants. The loss of labelled calcium 

 atoms in the transition from the second to the third generation 

 of one strain of mice amounted to 1/200 only. Calcium atoms from 

 the mother should no longer be traceable in the 12th generation of 

 these animals. 



That the calcium atoms of the ancestors can be traced in such a long 

 series of descendants can be ascribed to the fact that about 99 percent 



