RETENTION OF ATOMS OF MATERNAL ORIGIN IN THE ADULT WIIITK MOUSE 203 



about 188 days elapse; this corresponds to 13 half- lite periods of radio- 

 pliosphorus. During this time, the activity administered to the first 

 mouse has decreased to less than 1/8000 of its original value. As the 

 lourth generation may be expected to contain about 10~" of the 32p 

 present in the first generation (apart from the radioactive decay), we 

 may expect to find only about 10~ii of the radio-phosphorus adminis- 

 tered to the first generation. The measurement of such a low percentage 

 of radio-phosphorus administered would require the administration 

 of 100 millicuries ^^p or more. 



The phosphorus content of an adult mouse amounts to about 200 mgm 

 ()!• to 4- 1021 atoms. As of the ^^p atoms present in a mouse only 14 P^r 

 cent are found in a mouse of the next generation, it is easy to show- 

 that the eleventh generation will no longer contain a single ^^P atom 

 and thus no P atom at all which was present in the first generation. 



This result illustrates strikingly the fact that the hereditary disposi- 

 tions are entirely independent of any atomic community with the fore- 

 fathers, these dispositions being determined exclusively l)y the faculty 

 of the atoms and molecules to enter certain characteristic configurations. 

 It is the pattern which matters and not the single brick. 



Summary 



Labelled phosphate was administered to a pregnant mouse. Each offspring 

 was found to contain aknost the same amount of ^^^P. By kilhng the various offspring 

 at different dates and comparing their ^ap contents, we determined the amount 

 of maternal phosphorus atoms present in offspring at different times. 



Between birth and maturity, i.e. in the course of three months, the mouse 

 lost about 60 per cent of the P atoms acquired by birth. 



By breeding three generations of mice, to the first generation of which ^^V 

 Avas administered, the passage of phosphorus atoms from one generation to the 

 next was followed. 0.6 per cent of the ^~P present at the birth of an offspring 

 of the second generation was found to be present in an offspring of the third 

 generation. We might expect to obtain the same ratio between the ^'-P content 

 of the third and fourth generation, and so on. 



Making this assumption it can be shown that a mouse of the eleventh generation 

 no longer contains a single phosphorus atom present in the first generation. 



References 



Hevesy G. (1939) J. Chem. Soc. 1213. 



Hevesy G. C., Levi H. B. and Rebbe O. H. (1940) Biochem. J. 34, 532. 



