CYTOPLASMIC DNA IN IRRADIATED NEURAL TUBE 79 



tieated embryos, wiiile mitosis was inhibited for IV2 and 3 hours fohowing 

 the 200 and 500 r exposures respectively. Tritium, once added to the es:g, 

 remains axailable over a prolonged period, so that a high degree of incor- 

 poration results (Sauer and Walker, 1959. 1961 i. Assuming that the mitotic 

 stages are the ones most sensitive to radiation, x-ray afTects only those in 

 mitosis at the time of treatment. In the tritium-labeled embryos, howe\er, 

 within the course of a few hours practically e\ery neural tube cell would 

 undergo mitosis and thus be exposed to radiation at its sensitive stage. Those 

 embryos exposed to the combined action of tritiated thymidine and 200 r did 

 not appear to show much greater injury than those that had received thymi- 

 dine alone. 



With only light microscopy, there remained the possibility of error as to 

 the actual cytoplasmic location of the bodies. The electron microscope 

 pro\ed in\aluable in demonstration of electron dense bodies of complex 

 form not found in normal material 'Fig. 12 j. 



Discussion ^ 



Feulgen-staining granules located outside the nucleus ha\e been described 

 in many species under a wide variety of conditions. They occur normally, 

 especially in embryonic development 1 Gliickmann, 1951; Chang, 1940'. 

 but also in the adult in certain locations (Corner. 1932) : pathologically fol- 

 lowing cell death (Barthels and Voit, 1931) and in association with \iruses 

 (Leuchtenberger et al., 1956) : and experimentally in tissue cultmes of nor- 

 mal vertebrate embryos (Maximow. 1925) and in iriadiated material ' .'M- 

 berti and Politzer. 1924: \on Sallmann ct al., 1957 . Wherever encountered, 

 they resemble in their deep staining the chromatin of mitotic stages and are 

 surrounded by a portion of cytoplasm more deep staining than the 

 remainder. 



E.xtranuclear chromatin bodies assume prominence in developmental 

 stages in both plants and animals in connection with death of superfluous 

 cells, as in regression of transient structures or following excessive cell pro- 

 duction. The latter is probably an almost uni\ersal growth phenomenon. 



In the intensely studied field of insect dexelopment. Feulgen-positive 

 bodies regularly occur both within the cytoplasm and e.xtracellularly. Wig- 

 glesworth (1942) in an extensive review established two facts: a) whole 

 nuclei break down, and b ) the granules arc often intracellular. They are 

 most numerous during active mitosis, when excess cells would be formed. In- 

 corporation of the remnants of a dead cell by a neighboring cell seemed a 

 distinct possibility in epidermis with its intercellular connections: also, in 

 rapid cell division, of one of the daughter nuclei died before the cytoplasm had 

 divided, the dead cell would remain as a cytoplasmic inclusion. In one 



