188 INTRODUCTION TO CYTOLOGY 



single chromosome only, or even but a small portion of it, may be elimi- 

 nated or changed. Often a portion of a chromosome becomes detached 

 from the remainder {Jragmentation) , or attached to another chromosome 

 {translocation), or reversed in position {inversion), or rendered functionless 

 {inactivation) , or removed altogether {deletion). These and other altera- 

 tions, including changes in individual genes, may be brought about not 

 only during certain phases of mitosis but also when the nuclei are in the 

 metabolic condition, as in pollen and dormant seeds. This affords strong 

 support to the view that some characteristic organization is maintained 

 by each chromosome at all stages of the mitotic cycle. Treatments with 

 ultra-violet radiations^^ and the rays emitted from radium ^^ have also 

 been found to cause alterations of the protoplasm and of nuclear behavior, 

 some of which are comparable to those produced by X-rays. As will 

 appear in later chapters, it is such specific effects of irradiation, partic- 

 ularly those obtained with X-rays, which give the method a high value in 

 modern cytogenetic analysis. 



The discovery of "mitogenetic rays" emitted by growing tissues was 

 announced in 1923 by Gurwitsch,^^ who reported that when a growing 

 onion root (as "sender" of the rays) is pointed directly at the region of 

 cell-division in another root (as "detector") and left for some hours, 

 the detector shows more mitoses in the side toward the sender, the excess 

 over the other side being as much as 50 per cent. Metabolically active 

 tissues in considerable variety have since been reported to emit such rays, 

 and growing yeast cultures have come to be favorite detectors. Attempts 

 to determine the nature of the rays have led to the belief that they are 

 ultra-violet radiations of very low energy content, and that they may 

 possibly be associated with oxidation. 



The value of the evidence for the existence of such rays, or for a 

 causal relation between them and the effects observed, has been adversely 

 criticized by a number of investigators.^^ Reasons for discrepancies in 

 the results of various trials are beginning to be apparent in differences 

 in technique and in the condition of the materials used. At present it 

 appears probable that radiations produced by processes occurring in 



i« Takamine (1923), Schleip (1923), Addoms (1927), Just (1933), Heilbrunn and 

 Young (1930), Bucholtz (1931). 



1' Mohr (1919), M. Williams (1925a), Gager and Blakeslee (1927), Canti and Spear 

 (1927, 1929), Hanson and Heys (1928, 1929), Hanson and Winkleman (1929), Levine 

 (1929), Stein (1929), Stadler (19286, 19316), Stoel (1928), Goodspeed (19295), Good- 

 speed and Avery (1930). 



18 For general accounts, see Gurwitsch (1925, 1926, 1929, 1932), Reiter and Gdbor 

 (1928), and Hollaender and Schoeffel (1931). See also Wagner (1927, 1928), Magrou 

 and Magrou (1927), Baron (1928, 1930), Reiter and Gdbor (1929), Frank (1929), 

 Siebert (1928), Borodin (1930), Kisliak-Statkewitsch (1927), Loos (1930), Ferguson and 

 Rahn (1933), Tuthill and Rahn (1933). 



19 Guttenberg (1928), W. Schwarz (1928), Rossmann (1929), Taylor and Harvey 

 (1931), Richards and Taylor (1932), Moissejewa (1931). 



