264 L. H. Gray 



procedure itself occasionally resulted in pyknosis. Discrete 

 chromosome fragmentation, which arises from irradiation of 

 the intact oocyte, was not recorded as a result of the injection 

 of irradiated cytoplasm. 



Ord and co-workers (1952) have used nuclear transfers 

 between amoebae to evaluate the site of action of nitrogen 

 mustard and X-radiation when these agents give rise to 

 division delay and cell death. It was not possible to irradiate 

 isolated nuclei. Inferences were based on comparisons be- 

 tween amoebae treated whole and amoebae reconstituted 

 from an untreated nucleus and cytoplasm treated either in the 

 presence or absence of a nucleus. Although both agents are 

 lethal to the cell at considerably lower doses when the nucleus 

 is treated than when only cytoplasm is treated, important 

 differences between the effects of the two agents were noted. 

 With regard to radiation damage, it was found that damage to 

 the cytoplasm was of two kinds, a reversible damage which is 

 maximal at 100,000 rads., and lethal damage which becomes 

 prominent at 290,000 rads. This is, of course, a very high 

 dose. Exposure of an aqueous solution to this dose could 

 completely transform reactants at the millimolar concentra- 

 tion level. It is not surprising that it should be lethal to 

 cytoplasm. The comparatively high mean lethal dose for 

 nuclear damage (120,000 rads.) may be due to the occurrence 

 of each genetic factor at a high multiplicity (high polyploidy 

 or polyteny) as is almost certainly the case with Paramecium, 

 which has a radiosensitivity comparable with that of Amoeba 

 (Kimball, 1949). 



It was concluded that a dose of X-rays which is lethal to all 

 nuclei in the amoeba will not cause lethal damage to cytoplasm, 

 and that the nuclei were probably damaged independently of 

 the cytoplasm. 



Polyploid and Multinucleate Cells 



The study of polyploid and multinucleate micro-organisms 

 has thrown considerable light on the matter under discussion. 



