CELL DIVISION, MORPHOLOGY, VIABILITY 803 



Irradiation of isolated amphibian eggs with as little as 1000 r of X rays 

 is sufficient to affect the appearance of the nucleoli (Duryee, 1949). 

 They are changed from small, rounded bodies somewhat irregular in 

 shape and with small internal vacuoles to much larger bodies more nearly 

 spherical in form and with large internal vacuoles. Cattley (1909) 

 noted an increase in the number of nucleoli per cell in plant root tips soon 

 after X irradiation. Grasshopper neuroblasts subjected to a dose of 

 10,000 r of X rays at telophase exhibit shortly afterward several spherical 

 instead of the usual two nucleoli (Carlson and McMaster, 1951). Treat- 

 ment of neuroblasts at other stages of the mitotic cycle or at interphase 

 fails to alter the nucleoli. 



With regard to the means by which high-energy radiations act on the 

 nucleus, Duryee (1939, 1947, 1949) has made a strong case for an indirect 

 effect through the cytosome. Fragmentation of chromosomes and their 

 lateral loops and vacuolation and enlargement of nucleoli were used as 

 criteria of radiation damage. X irradiation of nuclei in situ or micro- 

 injection of the cytosome of nonirradiated eggs with cytoplasm withdrawn 

 from irradiated ones produced these changes. Immersion of isolated, 

 untreated nuclei in irradiated cytoplasmic brei led to loss of chromosomal 

 loops and heavy nucleolar damage. On the other hand, nuclei irradiated 

 with comparable doses after removal from the egg cytosome exhibited no 

 detectable injury. Duryee has concluded, on the basis of these results, 

 that the primary physical or radiochemical changes are produced in the 

 cytosome, that substances toxic to the nucleus accumulate in the cyto- 

 some, and that the subsequent movement of these toxins into the nucleus 

 effect the morphological changes seen after irradiation. 



Achromatic Figure and Cleavage. It has generally been found that only 

 very large doses of radiation, i.e., many thousands of r, affect the achro- 

 matic figure of the dividing cell. Henshaw (1940d, 1941) demonstrated 

 that 62,400 r administered to either gamete of Arbacia produced multi- 

 polar cleavage in almost 100 per cent of the zygotes. Polyspermy was 

 ruled out as a cause of this, for cytological examination showed that the 

 percentage of polyspermy was no greater in treated than control speci- 

 mens. He concluded, because multipolar cleavage was present after 

 treatment of the sperm, which contains almost no cytoplasmic material, 

 as well as the egg, which contributes no aster in fertilization, that the 

 supernumerary asters must result from an effect on nuclear rather than 

 cytoplasmic material. Unless one is willing to assume that the effective- 

 ness of the treatment depends on the quantity of cytoplasm irradiated or 

 that the sperm is devoid of cytoplasm exclusive of centrioles, the possi- 

 bility of an indirect effect through the cytoplasm is not eliminated com- 

 pletely. As much as 8000 r applied to the dividing grasshopper neuro- 

 blast has no demonstrable effect on the structure or functioning of the 

 spindle. 



