CELL DIVISION, MORPHOLOGY, VIABILITY 807 



lateral loops swell and disintegrate. (3) Nucleoli along the inner surface of the 

 nuclear membrane evert their contents into the cytoplasm, and their residual 

 shells swell and disintegrate. (4) The central nuclear ground substance . . . 

 changes from a gel to a sol, thereby allowing the chromosomes to tangle, mat, and 

 clump. (5) Later radiation damage with advanced pyknosis consists of forma- 

 tion of a clumped central nuclear body. 



Nuclear damage is induced in ovarian eggs of Triturus within 2 days at 

 22-24°C by doses of 2000 and more r of X rays applied to the whole body 

 of the female (Duryee, 1949). Experiments in which the ovaries were 

 shielded during irradiation showed that this was the result of direct 

 effects on the eggs and was not secondarily induced through the body of 

 the female. If the animals were kept at 4°C from the end of treatment 

 on, the pyknotic changes were temporarily arrested, but not for more than 

 about two weeks. After warming to 22°C, pyknosis appeared. Main- 

 tenance of animals at 27°C, however, after irradiation increased the rate 

 of nuclear disintegration. 



Lea (1946) has discussed at some length radiation-induced lethal effects 

 and their causes in higher organisms. From evidence based largely on 

 radiation of the Drosophila sperm, the Tradescantia microspore, and the 

 bean root tip cell, he has concluded that chromosome losses resulting from 

 asymmetrical interchanges and simple breaks are responsible for at least 

 some of the cell deaths that occur at or following division. It should be 

 pointed out, however, that the evidence from these organisms, in which 

 the analysis of chromosome changes is possible, cannot justifiably be 

 applied to the elucidation of evidence from cells that have been used in 

 the study of immediate lethal effects, which are manifest in the treated 

 cells before, during, or immediately following the first postirradiation 

 division of the cell. The test of a lethal effect induced in the Drosophila 

 sperm is failure of the egg fertilized by that sperm to hatch. Chromo- 

 some losses might be expected to lead to faulty differentiation and death 

 of the embryo, but this appears to me to be a quite different cause of 

 death from the immediate effect that would result in the death of the 

 zygote or one or both of its daughter cells. The same is true of the 

 bean root-tip cell where the test of lethal effect is the death of the root 

 many cell generations and many days (about 14) after treatment. The 

 one example cited by Lea of an immediate lethal effect is the failure of the 

 irradiated Tradescantia microspore to differentiate or failure of the pollen 

 tube to develop. The microspore, however, is a haploid cell and any 

 chromosome loss might be expected to produce an immediate lethal effect. 

 The chick fibroblast, the cells of the brain and eye of the tadpole, malig- 

 nant cells of the mouse, and rat retinal cells are not haploid. Unless both 

 members of a chromosome pair suffered the loss of corresponding regions, 

 and this would be a rare event except after large doses, there is no reason 

 to think that the loss would cause the death of the cell. It seems to me, 



