PROTOZOA AND INVERTEHRATE EGGS 291 



the ability of the cells to produce cultures, or infections in the case of 

 parasitic forms, is investigated, death will be due, obviously, to the most 

 sensitive of these processes occurring within the part of the life cycle 

 investigated. 



Sensitivity. The sensitivity of individual cells of the same species to 

 the lethal effects of radiation varies considerably. Halberstaedter and 

 Back (1942) found that all cells in a colony of Pandorina died at the same 

 dose although different colonies required different doses. Secondly, they 

 found, by giving repeated increments of 100,000 r and examining for 

 immediate death after each increment, that the colonies of a small clone 

 (16 colonies) all died at the same, or approximately the same dose; but 

 colonies of different clones died at quite different doses. In this way some 

 clones were found which died at 300,000 r and others which died at 

 600,000 r. 



Back and Halberstaedter (1945) demonstrated a somewhat similar 

 phenomenon with Paramecium caudatum. When groups of about eight 

 paramecia from cultures derived from a single animal by at least six 

 divisions were tested, it was found that the eight might die at quite 

 different doses. However, the four to eight animals derived from one, by 

 two or three divisions, were found to die almost invariably at the same 

 dose, although different groups died at quite different doses. Therefore, 

 the sensitivity is the same for closely related individuals but not for the 

 more distantly related ones. 



This investigation of Back and Halberstaedter is of much interest, for 

 it suggests that minor variations between cells may lead to rather marked 

 changes in sensitivity. The evidence also suggests that these minor 

 variations may be maintained over a few cell divisions. It is improbable 

 that the similarity between products of a single Paramecium could be due 

 to the products being in the same part of the fission cycle, for the numbers 

 given in the tables suggest that Back and Halberstaedter (1945) often 

 used groups in which some of the animals had divided once more than 

 the others. 



Different stages in the life cycle of a given species may be of different 

 sensitivity. Tang and Gaw (1937) find that older cultures of Paramecium 

 bursaria are more susceptible to the immediate lethal effects of ultraviolet 

 than younger ones. Brown et al. (1933) report that cysts of the ciliate 

 Ewplotes taylori are killed by approximately 400,000 r of X rays, whereas 

 the motile form requires approximately 460,000 r. The criterion in this 

 case was the ability to survive 48 hr. However, they suggest that this 

 difference may have been due to the different media in which the cysts 

 and motile forms were kept. Bennison and Coatney (1945) found that 

 8000 r of X rays prevented infection of chicks by a suspension of sporo- 

 zoites oi Plasmodium gallinaccum, while 20,000 r was required to prevent 

 infections by suspensions of trophozoites. Here, as in the work with 



