480 JOHN W. G O W E N 



ness, placed on slides, stained, and studied microscopically. The 

 process is not simple. Texts on histological technique describe the 

 methods {19). 



A more rapid method is the smear method (74). in which the tissue 

 for the study of the chromosomes is dissected out, placed on a slide 

 in a small drop of acetocarmine or aceto-oi'cein, and then pressed to a 

 single cell thickness by pressure on a cover glass. Such preparations 

 may be made permanent by replacing the water and acetic acid in the 

 acetocarmine mount by alcohol, xylol, and balsam or euperol. 



Since cytological studies have generally sought the chromosome 

 changes produced by the X rays, the tissues studied for these X- 

 ray effects have been confined to those showing good chromosomes. 

 These tissues are the germ cells — in plants, the pollen mother cells, 

 pollen tube nuclei, etc.; in animals, the testis or ovaries (one sex 

 or the other being preferred in a given species), ganglion cells, and 

 salivary gland cells. Detecting such chromosomal changes as chro- 

 mosome breaks and rearrangements necessitates familiarity with the 

 geometrical figures formed by these chromosomes when they are so 

 rearranged. The extensive nature of these changes makes it neces- 

 sary to refer to text diagrams (see 3). 



Drosophila chromosomes as found in the salivary glands have the 

 important advantages of large size and consistent arrangement of 

 some 5000 bands along these chromosomes. These bands accurately 

 locate chromosome breaks within the chromosomes. The types of 

 breaks may be identified (32) . Chromosomes show loss of small sec- 

 tions as deficiencies in the bands. The deletions of large sections are 

 generally lethal, so that these figiues are not observed cytologically, 

 although they do contribute materially to the deaths of the irradiated 

 Drosophila. Inversions of chromosome sections are observed by the 

 fact that in the inverted sections the chromosomes do not pair 

 properly. Similarly, the intei'change of chromosome parts from one 

 chromosome to another is obser\'ed as (crossed figures where breaks 

 occur. By classifying the data into these categories it is possible to 

 specify the X-ray action as due to one, two, three, etc. absorptions 

 of the X-ray energy to which the material may be exposed. 



4. Bacterial Mutatioiis 



X-rays are frecjuently used to speed up the mutation rate of the 

 genes affecting particular characters in the hope of obtaining favor- 



