PROTOZOA AND INVERTEBRATE EGGS 287 



tive, whereas 2537 A was less effective. McAulay and Taylor (1939) 

 investigated the death (by bursting) of Paramecium as a result of exposure 

 to monochromatic ultraviolet. They found that all wave lengths up to 

 3000 A were quite effective but that effectiveness was much less at longer 

 wave lengths. It may be doubted that their techniques were sufficient 

 to detect differences in effectiveness among the shorter wave lengths. 

 Shettles (1938), using the flagellate Peranema trichophorum, observed 

 death after 2 hr exposure to 2537 A ultraviolet but no death after 8.5 hr 

 exposure to 2650 A. Likewise, no death was found after comparable 

 exposures to longer wave lengths. The intensities of the different wave 

 lengths are said to have been eciualized by use of a photometer. Thus 

 no general conclusions are possible about wave-length dependence of 

 immediate death after ultraviolet. 



It is probable that immediate death is due to extensive damage to the 

 cellular material. This is suggested by both the rapidity with which it 

 can occur and the very large doses of ionizing radiation necessary to bring 

 it about. It might be surmised from the microscopic observations that 

 coagulation of the protoplasm or damage to the cell membrane, or per- 

 haps both, are involved, but no really conclusive evidence is available. 

 Possibly, different kinds of damage lead to death in different cells under 

 different circumstances of irradiation. 



It is rather generally accepted (Lea, 1947) that irradiated cells may be 

 able to survive until they divide, at which time death is caused by loss of 

 parts of chromosomes as a result of chromosome aberrations induced by 

 the radiation. Such a mechanism is suggested by Hoi week and Lacas- 

 sagne's observation (1931a, b) of death at division in the flagellate 

 Polytoma. At certain doses of a. particles, many of the cells remained 

 normal until division and then disintegrated. Using the hit theory, they 

 calculate that a two-particle event is involved with a target diameter of 

 2.3 ju, corresponding roughly to the area presented to the particles by the 

 pericaryosomal space in the nucleus, the region in which, presumably, the 

 chromosomes are located. Similarly, Halberstaedter and Back (1942) 

 report that Pandorina colonies exposed to between 3000 and 300,000 r 

 remain normal until cell division, when cytolysis takes place. This was 

 so regardless of whether old colonies that underwent cell division shortly 

 after irradiation or young colonies that did not undergo cell division for 

 some time were used. 



When the test of survival is the ability of the cell to multiply and pro- 

 duce a culture, death due to mutations or chromosome aberrations must 

 be considered as a possible explanation, although other causes cannot be 

 excluded. Two cases showing the ability of free-living flagellates to 

 produce cultures after X irradiation may be mentioned. Ralston (1939) 

 distinguishes immediate from delayed death and reports the median lethal 

 dose foi- delaved death in Dunaliella salina as between 10,000 and 13,000 r. 



