CHROMOSOME ABERRATIONS IN ANIMALS 



09 1 



mitosis cannot at this time be clearly denned. Movements of chromo- 

 somes incident to the coiling of the chromonemata and formation of the 

 equatorial plate may be of importance in modifying the reaction system. 

 As mitosis progresses, nuclear membrane and nucleoli normally disappear 

 and are later re-formed. Profound changes are thus effected in the 

 structure of the chromosome and its cellular environment. Chemical 

 changes that have been detected within chromosomes and nucleoli in 

 the course of these events involve the types, proportions, and patterns of 

 association of nucleic acids and proteins. Very little is known about the 



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STAGE IRRADIATED 

 A SCORED AT ANAPHASE I • TOTAL FRAGMENTATION INDUCED 

 O SCORED AT MICROSPORE METAPHASE 

 Fig. 9-17. Graph showing the numbers of fragments induced by 50 r of X rays at 

 various stages of the nuclear cycle during microsporogenesis in Trillium erectum. 

 (From Sparrow, 1951.) 



cyclic alterations in associated cellular materials. Structural and func- 

 tional modifications of cellular components may be effected by a multi- 

 tude of physical and chemical agents (listed in Sparrow, 1951). To the 

 extent that such modifications affect the reaction system initiated by ion- 

 izing radiations, the agents used will alter the radiosensitivity of cells. 



5. CHEMICAL AND CYTOCHEMICAL STUDIES 2 



Numerous attempts have been made in recent years to correlate the 

 cytologically detectable effects of ionizing radiations with changes in 



2 This section has been prepared in collaboration with Miss Helen Gay, to whom the 

 author expresses his sincere thanks. 



