Sizova (1936) observed the structural trans- 

 formation of chromosomes under the influence 

 of X radiation of physiologically changed cells. 

 Preliminary experiments showed that no struc- 

 tural changes were induced in Crepis capillaris 

 sprouts which had been kept in a desiccator filled 

 with carbon dioxide for 5 hours. Roots kept for 

 5 hours in carbon dioxide before irradiation 

 showed 50% of their plates changed after irradi- 

 ation. Those which had not been in carbon 

 dioxide showed changes in only 26% of the plates. 

 However, the chromosomal changes were simi- 

 lar in character. Ammonia fumes also fail to 

 produce structural chromosomal changes, but 

 the chromosomes become shorter and thicker. 

 If, after being kept in ammonia fumes for 6 

 hours, the sprouts are irradiated, the number 

 of changed plates reaches 44%. A preliminary 

 cooling of the rootlets by ice before radiation 

 has a similar effect. The number of plates 

 with structural changes then goes up to 40%. 



Marshak (1937) investigated the frequency of 

 abnormal chromosomes due to X rays at ana- 

 phase. He performed his experiments on a 

 wide variety of objects: sarcoma of mice, 

 carcinoma of rats, and rootlets of Vicia faba, 

 Pisum sativum . Allium cepa, and Lycopersicum 

 esculentum . In all objects the frequency of 

 chromosomal abnormalities (chiefly fragmenta- 

 tion and fusion of chromosomes) reaches its 

 maximum 2 or 3 hours after irradiation and 

 then gradually returns to normalcy after 48 

 hours. The curves which describe the action 

 of X rays are identical for all objects. The 

 curves of the retardation of mitoses are parallel 

 to the curves of chromosomal abnormalities. 

 According to Marshak' s observations, the 

 retardation of mitoses is not related to the 

 length of the chromosomes as some investigat- 

 ors had supposed. Three hours after irradiation 

 by equal doses, frequency counts of changed 

 anaphases showed 19% retardations in Allium , 

 which has extremely long chromosomes, and 

 22% in Lycopersicum , whose chromosomes are 

 among the shortest in the plant kingdom. De- 

 spite the great differences in chromosome 

 length of the various objects investigated, the 

 diameters of their sensitive areas are surpris- 

 ingly similar in their dimensions. This simi- 

 larity causes us to suppose that this is a di- 

 mension of some physical structure in the 

 chromosome, which is identical in all of the 

 objects investigated. The size of these diam- 

 eters coincides with the average diameter of 

 polypeptid chains or the diameter of a molecule 

 of a protamine (10 x 10' 8). Similarly, the 

 reaction to the action of ammonia and carbon 

 dioxide before and during irradiation agrees 

 with the hypothesis that the sensitive substance 

 of the chromosomes belongs to the same cate- 

 gory of protamines and histones. Cyclical 

 changes in localization and distribution of the 

 chromatin material indicate that this substance 

 is negatively charged and apparently consists 

 of nucleic acid. These changes in the distribu- 



tion of the negatively charged substance are in 

 accord with the observed changes in sensitivity 

 to X rays. 



Sax (1938) irradiated the microspores of 

 Tradescantia reflexa, which had six pairs of 

 chromosomes and one pair of fragments. Sev- 

 eral hours after irradiation by a dose of 25 r^^ 

 the meiotic chromosomes as well as the chro- 

 mosomes in the microspores clump and the 

 homologous chromosomes^^ fuse. Twenty- 

 four hours after [microspore] irradiation 

 bridges and free fragments are observed at 

 anaphase. After 48 hours. Sax observed sev- 

 eral breaks in the chromosomes, but 4 days 

 after irradiation he found none. The breaks 

 release the distal end of the chromosome arm 

 and the broken ends of sister chromosomes 13 

 fuse at one end. The first break of the chromo- 

 some usually is observed 6 hours after irradia- 

 tion. When the broken chromosome divides at 

 anaphase, its ends fuse, forming a bridge. 

 The distal ends of the broken chromatids always 

 fuse to form a single fragment. The size of 

 the fragments can vary, but bridges never exist 

 without fragments. The break in two chromo- 

 somes may be followed by a reciprocal inter- 

 change, or by fusion of the chromosomes with 

 the release of a fragment. It is very difficult 

 to trace such reciprocal interchanges. As has 

 been already pointed out, the broken ends of 

 each arm of a single chromosome can form 

 chromosome rings. Irradiation of meiotic 

 chromosomes can produce a high rate of ster- 

 ility in the microspores; nevertheless, some 

 of the microspores do develop, and they show 

 very frequent chromosome breaks. If the 

 fragments are not lost, the chromosomes from 

 which they broke off develop normally. Some- 

 times, as a result of irradiation, diploid micro- 

 spores develop, and frequently microspores 

 with chromosomal aberrations appear (frag- 

 mentations, fusion of chromosomes, and, more 

 rarely, unequal distribution between the poles, 

 monocentric spindles, and inactivity of the 

 centromeres). In contradiction to the opinion 

 expressed by many cytologists that the "hit" 

 delivered by X rays can break only one chro- 

 mosomei4 at a given locus. Sax considers that 

 his observations demonstrate breaks in chro- 

 mosomes and in chromatids in the same divi- 

 sion figure, and even in the same chromosomes. 

 Then he sets out to determine whether these 

 breaks are localized or not. If the break and 



'^Editor's note: text in error; Sax (1938) states: 

 "the tube delivered about 25 r per minute. The 

 dosage used ranged from 75 to 200 r . . . " 



"Editor's note: text should read "chromatids" 

 according to Sax (1938). 



"Editor's note: text should read "chromonema" 

 according to Sax (1938). 



73 



