CHROMOSOME ABERRATIONS IN ANIMALS 629 



salmon; Lasnitzki, 1943b, 1948, on avian tissue cultures; Pfuhl and 

 Kiintz, 1939, on connective tissue cells of rabbits; Roller, 1947, on 

 normal and malignant cells of man). A few selected illustrations are 

 presented in Fig. 9-1. Alberti and Politzer realized that pycnotic chro- 

 mosome masses represent one aspect of cellular necrosis. When alter- 

 ation of the chromosomes is so complete that individual members of the 

 set cannot be recognized, the aberrations are not serviceable for quanti- 

 tative studies of breakage, but are useful, as will be shown presently, 

 for chemical and cytochemical analysis of the changes effected in the 

 nucleic acids and proteins that represent the major organic constituents 

 of chromosomes. 



Of greater use in quantitative studies are those "secondary" or 

 genetic" effects induced by moderate doses of radiation, in which 



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Fig. 9-1. Abnormal mitoses, with adherent chromosomes resulting from exposure of 

 cells to ionizing radiations, (a) (b) From grasshopper neuroblasts (Carlson, 1941b); 

 (c) (d) from malignant cells of man (Roller, 1947). 



damage is sufficiently localized to break the chromosomes without impair- 

 ing permanently the synthetic and reparative processes essential to 

 mitotic and other vital cellular activities. Under these conditions the 

 chromosomes may establish new associations by union of their broken 

 ends. The resulting rearrangements can be detected by examination of 

 the treated cells or their descendants, or by genetic analysis of individuals 

 carrying the aberrations. Production of such viable chromosomal 

 exchanges by X-ray treatment was first reported by Muller (1928a, b) 

 and Muller and Altenburg (1928, 1930), who designed excellent methods 

 for detection and preservation of the induced rearrangements. These 

 furnished a wealth of experimental material, whose analysis during the 

 second quarter of this century has greatly furthered understanding of 

 the mechanisms of heredity and evolution. Determination of the fre- 

 quency of these gross chromosomal aberrations under various experi- 

 mental conditions has also furnished basic data for a preliminary analysis 

 of the processes involved in chromosome fragmentation and reconstitu- 

 tion. The significance of such data in supplying information about the 

 mode of action of ionizing radiations and the possible control of deleteri- 

 ous effects has become increasingly apparent in recent years. 



