94 Applied Biophysics 



to insure constancy in age and temperature of the biological 

 indicator selected. ^-^ 



Among the materials used in this way, the following may be 

 mentioned, although not all conform to Holthusen's specification 

 for the ideal test-object : Eggs of the sea-urchin, Ascaris, Droso- 

 phila, silkworm, grasshopper, frog and axolotl, viruses, bacteria, 

 yeast, pollen grains, protozoa, vegetable root-tips, and tissue 

 cultures. ^^' ^'^' ^^' ^^"^ Germ cells and somatic cells of higher ani- 

 mals, blood cells, skin, and even whole animals have also served 

 as indicators in special cases. ''^- 



Such material has been used for demonstrating the wide dif- 

 ference in sensitivity which exists among biological objects. 

 This is illustrated, for the lethal efifect, in Table II, taken from 

 data given by Packard ^^^ and by Crowther.^^ The reason for 

 these great differences is quite unknown. 



TABLE II 



Dose in Rontgens Necessary to Kill 50% of the 



Samples of Organisms Irradiated or to Reduce 



Their Growth to Half That of Controls 



Organism Dose in r 



Eggs of CaUiphora 40 



Eggs of Axolotl 50 



Eggs of Drosophila 190 



Eggs of Ascaris 1,000 



Larva of Drosophila 1,300 



Escherichia colt 5,100 



Mesotaenium 9,000 



Saccharo)iiyces 42,000 



Imago of Drosophila 95,000 



B. mesentericus 200,000 



Colpidium colpoda 330,000 



Biological indicators have also been extensively used in studies 

 of the efifect of wave length on biological response, in genetics, 

 and in testing the validity of various theories of action of radia- 

 tion and the significance of alterations in the physical conditions 

 of irradiation. 



The results, although usually consistent for a given material, 



