PROTOZOA AND INVERTEBRATE EGGS 289 



relatively well buffered against radiation damage and, in a sense, this is 

 true. The continued reproduction of the flagellates is prevented by a 

 few tens of thousands of roentgens of X rays, but hundreds of thousands 

 are needed to prevent the vegetative multiplication of ciliates. Back 

 (1939) found for Paramecium caudatum that a dose about two-thirds the 

 immediate lethal dose, i.e., about 300,000 r, led to eventual death. The 

 animals survived for several weeks but decreased in size, and finally died. 

 Lacassagne (1934b), working with Glaucoma scintillans, had previously 

 reported death after several days without division. However, he found 

 that some increase in size occurred. Thus delayed death does occur in 

 these organisms but it occurs without division. 



Kimball and Gaither (1951) have been able to distinguish at least two 

 kinds of delayed death in Paramecium aurelia following exposure to ultra- 

 violet. With higher doses, some of the animals survive a day or more 

 without division, but eventually die. Long periods of the kind reported 

 by Back (1939) for X rays were not observed for death without any divi- 

 sion. However, at slightly lower doses, the animals pass through two or 

 three divisions rather slowly (in 2 or 3 days) but then cease dividing 

 entirely for several days. Some animals finally recover the normal divi- 

 sion rate after remaining undivided for periods as long as three weeks, but 

 others die during this "cessation" period. All animals, whether they 

 eventually recover or die, become very small and thin during this cessa- 

 tion of division, and apparently all come very near death. From the 

 decrease in size of the animals, it seems probable that the delayed death of 

 Paramecium involves an effect on the synthetic processes within the cell. 

 Death could well result when the cell, no longer capable of making new 

 material, comes to the end of its resources. In the case of ultraviolet, 

 several divisions ordinarily elapse before the synthetic processes come to 

 a halt, while with X rays. Back's study (1939) suggests that this is not 

 so. Also, recovery has been found for the ultraviolet effect but no 

 recovery has been reported for X rays. Whether these differences are 

 real or only apparent remains for future investigation. In any case, it 

 seems hardly possible that death during the cessation period after ultra- 

 violet irradiation is due to mutational changes, since recovery can occur 

 even in the sister animals of those which die. It also seems improbable 

 that enough genetic damage could be done either by X rays or ultraviolet 

 to cause the delayed death without division, although perhaps such an 

 explanation is just possible with the large doses of X rays employed. 

 Nonetheless, death due to mutational changes is known for Paramecium 

 aurelia. Animals given doses of 10,000 r or less divide normally as long 

 as they multiply vegetatively, but when they undergo the self-fertilization 

 process of autogamy many of the exautogamous clones are inviable. This 

 phenomenon will be discussed in more detail in the section on inherited 

 effects. 



