230 ADVENTURES IN RADIOISOTOPE RESEARCH 



Here, the mean percentage uptake was found to be 56. From the above 

 data it follows that, out of the daily uptake of 8 mgm calcium by our 

 mice, at least 4 mgm. have passed the circulation, representing a mini- 

 mum amount of 2 mgm in the course of 500 days. From our results it 

 thus follows that these 2 mgm were prevented from interchanging with 

 2/3 of the 370 mgm calcium present in the skeleton of a mouse weighing 

 36 gm. The protected part of the skeleton calcium did not come into 

 contact with the plasma or lymph and, correspondingly, an exchange 

 between the unlabelled food calcium and labelled skeleton calcium could 

 not take place; the same is true for the new-formation of the protected 

 apatite crystals of this part of the skeleton under participation of food 

 calcium. A possible rearrangement within the protected area would 

 not manifest itself in our experiment. 



The inaccessibility of parts of the skeleton minerals manifests itself 

 also by the observation that radium, which like calcium is a strongly 

 bone-seeking element, can find a life-long abode in the skeleton. The 

 fact that a large fraction of radium administered to human subjects 

 remains for decades in the skeleton is due presumably to the incorpora- 

 tion of the radium into parts of the skeleton which are covered by apatite 

 layers and thus become inaccessible and, even if released, are incorpo- 

 rated again with the apatite structure. Aub and associates'^"' report 

 a case in which no decrease in the radium content of a woman was found 

 to take place between 1934 and 1945. This woman had been administered 

 radium in 1924. 



CONSERVATION OF ANCESTORAL ATOMS 



The radiocalcium atoms going over from the first generation of mice 

 into the second (cf. p. 226) do not indicate the total amount of maternal 

 calcium atoms passing from the mother to the offspring, since the mother 

 is not uniformly labelled. Chemical data indicate a passage of about 

 1 .3 per cent. Since the calcium of the second generation is uniformly 

 labelled, the passage of the ancestoral calcium atoms from the second 

 into the third generation is properly indicated by the radioactive tracer. 

 Aliout one third of the *^C'a content of the second generation is lost 

 prior to gestation, while about 0.5 per cent or less of the remainder 

 passes into the third generation. From the calcium atoms present at 

 birth in each generation, thus 1/300 P^''^ °^" ^^'"^^ §°^^ over to the following 

 generation. As our mice contained 6 . lO'-i calcium atoms, the eleventh 

 generation did no longer contain a single ancestoral calcium atom. 



It is of interest to compare the life cycle of the ancestoral calcium 

 atoms of the mouse with that of easily accessible water molecules. 

 Applying deutoriatecl or tritiated water as an indicator first the half 



