MAMMALIAN CELL GROWTH IN TISSUE CULTURE 187 



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Figure 4. Somatic chromosomes of a human cell, showing the sex chromo- 

 some and the six satellited members. 



anism of sex determination of man has been demonstrated by these 

 studies to have definite differences from that of Drosphila, the animal 

 previously considered to be a model system in this connection. 



These techniques have made possible a new approach to the prob- 

 lems of mammalian cell radiobiology. Because methods utilizing 

 massive cell populations tend to confuse the reversible delay in repro- 

 duction with irreversible killing, mammalian cells have often been con- 

 sidered to be resistant to irreversible radiation damage until doses of 

 thousands of roentgens had been administered ( Pomerat, 1958; Stroud 

 and Bruez, 1954 ) . Hence the mammalian radiation syndrome has often 

 tended to be interpreted as largel>' due to effects other than the repro- 

 ductive death of the individual somatic cells, and cellular genetic 

 damage has tended to be discounted as an apprecible factor in mam- 

 malian radiobiology ( Mole, 1959 ) . However, when the quantitative 

 methodologies here described were applied to this problem, it was 

 demonstrated that the mammalian somatic cell is one of the most radio- 

 sensitive cells yet studied. The mean lethal dose for the reproductive 

 function of all mammalian cells so far reported is included within the 



