CYTOPLASMIC DNA IN IRRADIATED NEURAL TUBE 77 



the end of 34 hours in any of the 200 r series, and by 57 hours recovery 

 seems to be complete. Ahhough a variable amount of degeneration occurs 

 even at this lower dosage, manifested in the neural tube as scattered nuclear 

 fragments or as areas with an indistinct luminal margin, there is no whole- 

 sale degeneration, and most nuclei remain essentially nonnal in appearance 

 (Figs. 2-8). To what extent recovery consists of reversal of the process in 

 the individual cell rather than replacement by unaffected cells can not be 

 answered by this study. 



Quite different results follow a dosage of 500 r. At 20 hours after exposure, 

 numerous inclusions fill many cells, and there is considerable cell death. 

 Debris is present in the \entricles of the brain and in the central canal of 

 the neural tube. This series shows considerable reduction in the mitotic rate 

 during the 20 to 30 hour period following treatment, while no such sec- 

 ondaiy delayed period of mitotic arrest follows the 200 r dosage. The 

 series receiving 500 r shows complete recoxery at 76 hours. 



From the small series of older embryos i4 to 5 days i receiving the same 

 amount of radiation as the two groups just described, it was determined 

 that age is not a factor in the mere appearance of inclusions, although their 

 distribution and time of maximum development differ in the two stages. 



Table I summarizes the staining reactions of most of the cytoplasmic 

 inclusions. Each body commonly displays a deep staining region, or center. 

 (Figs. 2 and 10) which is strongly basophilic and appears to contain a high 

 concentration of DNA. Both with Feulgen's stain and methyl green-pyronin, 

 the centers give a strongly positive reaction for DNA ; the negative reaction 

 when DNase precedes the staining confirms the DN.A. content of the 

 centers. 



Alfert I 1955; Vendrely ct al., 1958 i showed that reaction to methyl green 

 is not a reliable indicator of the degree of polymerization of DNA, as held 

 by Kurnick 1955a.b!. The intimate imion of DNA and protein in normal 

 chromatin keeps many groups imaxailable tor dye binding. Although the 

 DN.\ of pyknotic nuclei does not decrease until the fragmentation stage, 

 the intense yreen which pyknotic nuclei display can not reflect the amount 

 of DNA present, but only means that autolytic changes accompanying pyk- 

 nosis have unmasked stainable groups. 



The staining reactions carried out ( Table I ) support the concept that 

 these bodies also contain ribonucleic acid (RNA ) . With Kurnick"s 1955a, b) 

 modification of the methyl green-pyronin stain, for which he claims speci- 

 ficity for each of the types of nucleic acid, the body stains with a green 

 center surrounded by a red periphery. The color resembles the deep green 

 of the metaphases rather than the lighter green of other nuclei. The red color 

 of the periphery might mean either RN.A or depolymerized DNA. The 

 negative Feulgen stain and the absence of red color when the stain is applied 



