202 ADVENTURES IX RADIOISOTOPE RESEARCH 



still more jealously than it does the maternal P atoms. Similar conside- 

 rations may apply to the magnesium and fluorine content of the organism. 

 The atoms of maternal origin of all other elements present in the organism, 

 however, can to be expected to leave the body at a much higher rate 

 than do the phosphorus atoms of maternal origin. 



THE PERCENTAGE OF 32p TRANSFERRED FROM ONE GENERATION 



TO ANOTHER 



Some experiments were carried out in order to follow the fate of the 

 ^^P administered to a mouse in the second and the third generation. 



The phosphorus content^ of the new-born mouse constitutes about 

 2.4 per cent of the mother's. phosphorus content; consequently, we should 

 expect to find about 2.4 per cent of the ^^p contained in the pregnant 

 mother in each new-born mouse. Actually, only about 1/4 of that amount 

 is found when comparing the activity of a mouse of the third and the 

 second generation. This finding is explained mainly by the fact that 

 a large part of the ^-P content of the mother is to be found in the skeleton 

 and, as a large part of this ^sp Joes not reach the circulation, it does 

 not participate in the formation of the foetus. The foetus acquires its 

 phosphorus content mainly from the food phosphorus and the phos- 

 phorus present in the soft tissues. The phosphorus present in the circu- 

 lation has a much smaller ^ap content than the average phosphorus 

 of the mother, a fact which results in a comparatively low ^^p content 

 of the new-born mouse. The first generation contains labelled phosphorus 

 mainly in the soft parts of the body and in a minor part of the skeleton, 

 and this because it obtained its phosphorus content by subcutaneous 

 injection and not by the much more intimate foetal processes. The 

 32p content of the first generation is therefore not strictly comparable 

 with that of the second generation. The ^^p contents of the second and 

 the third generations may, however, be compared. We may expect 

 this ratio to be equal to that which we would obtain on comparing the 

 32P contents of mice of the third and fourth or of the fourth and fifth 

 generations etc. since it is to be expected that the percentage of ^ap 

 passed from the third to the fourth generation will be the same as that 

 transferred from the second to the third generation, etc. This conclusion 

 is important since it is almost impossible to follow the fate of ^'-P through 

 more than three generations of mice. 



From the injection of labelled phosphate to a mouse in the last stage 

 of pregnancy (first generation) to the birth of tho fourth generation 



1 The P content of a new-born mouse weighing 1.33 gm average value amounts 

 to 4.2 mgm while that of a movise weighing 31.8 g to 177 mgm 



