260 F. STEPHEN VOGEL 



changes in the intracelkilar compartments. It seems most likely that these 

 changes are manifestations of a rapid transferal of electron-lucent "nuclear 

 sap" from the karyoplasm into the perikaryon. Such volumetric changes 

 might result from alterations in the permeability of the nuclear membranes, 

 from hypotonicity in the nuclear chamber, which causes an egress of fluid, 

 from hypertonicity in the cytoplasmic compartment, which attracts fluid, or 

 from several of these factors acting simultaneously. The expansion of the 

 cytoplasmic space, which in many cells notably exceeded the volumetric 

 decrease in the nucleus, and the increase in the over-all size of the cells make 

 it seem likely that extracellular fluid is also imbibed by the perikaryon. Con- 

 sidered together, the findings suggest that radiation induces a marked hyper- 

 tonicity of the cytoplasm that, in turn, initiates a series of cytologic changes 

 that are transitory in the granule cell neurons of rabbits and other animals, 

 but often lead to cellular death in those of the dog. 



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