CONSERVATIOX OF SKELETAL CALCIUM ATOMS THROUGH LIFE 231 



life of water molecules present in the rat was found to vary between 

 3.6 and 2.5 days*^-'"''' '^^^ that of the mouse is expected to be somewhat 

 shorter. Thus, in the course of 165 days, all 10^4 water molecules present 

 at the start of the experiment in the mouse are replaced. About 4 per 

 cent of the maternal wattn- molecules go over to the offsprings and, 

 from these, the second and third generations of offsprings will take up 

 a share which depends on the age of gestation; the fourth generation, 

 however, will hardly contain any more ancestoral water molecules. 



When the rate of disappearance of labelled water was followed in the 

 rat during a long period, which was made possible by using tritiated 

 water as an indicator, it was observecP'^^ that, after the lapse of 30 days, 

 the labelled water disappeared at an appreciably slower rate than with 

 a half- life of 2.5 days. The controlling factor of the dissappearance of 

 labelled water from the organism is now the release of firmly bound 

 tissue tritium which again becomes a constituent of the water molecules. 

 Due to this fact, it lasts 60 days until the number of labelled water 

 molecules of this type, present in the mouse, decreases to a 10""^th of 

 its initial value. 



If we disregard those water molecules whose hydrogen atoms were 

 temporarily incorporated in tissue constituents and released appre- 

 ciably later to become constituents of water molecules again, then all 

 ancestoral water molecules are lost by the mouse during two generations. 



While the loss of ancestoral calcium is determined mainly by the loss 

 at birth, many ancestoral water molecules are lost during the lifetime 

 of a generation, none of them reaching the third generation of offsprings. 



Summary 



Since it was desirable to obtain uniform labelling of all calcium present in the 

 .skeleton of the mouse, ^^CaClg was added to all water administered to mice for 

 weeks before and after gestation. Such water was also given to new-born mice 

 after weaning until adult age was reached. The members of the litter, having 

 almost the same radiocalcium content, were then sacrificed at different dates 

 within 560 days. 



From the labelled calcium atoms present in the skeleton of the outgrown 

 mice, 67.2 ^b '^•9 P^r cent were found still to bo present in the skeleton of sister 

 mice sacrificed after the lapse of 390 days. 



When administration of ^^Ca was interrupted after the birth of the litter, and 

 its members reared by inactive mothers were sacrificed at different dates within 

 560 days, a mouse killed shortly after birth contained 8 per cent of the maternal 

 ^^Ca atoms, another mouse killed after 510 days contained 4 per cent. Half of the 

 calcium atoms present at birth is thus conserved during the lifetime of the mouse. 



From the figures obtained of the passage of labelled calcium from one geneia- 

 tion to the next, it follows that the eleventh generation does not contain a single 

 calcium atom present in the first generation of its ancestors. 



