XIV. X RAYS AND X I R R A D I A T I O N 477 



SO that the different factors affectinj^ variabiHty of th(> results may be 

 measured and taken into account. 



Some experiments require the irradiation of only the female in- 

 heritance. This may be done by irradiating the eggs just after the 

 last oogonial division when they are still retained within the body of 

 the virgin Drosophila female. The mature virgin female has 100 to 

 150 of these egg cells in her ovary. These eggs are in the diploid con- 

 dition, as reduction division in the Droi^ophila takes place only after 

 the sperm have entered the eggs. Immediatel.\' after irradiation, the 

 females should be mated to active males. The results will be biased 

 by effects produced by gene or chromosome rearrangements on the 

 maturation divisions. Comparison of the hatchabilities and other 

 changes induced by the different treatments with those of the controls 

 will give a measure of the X-ray effects. 



Treatment of the haploid male gametes is fairly easy in l)oth ani- 

 mals and plants, i.e., in Drosophila the sperm may be irradiated as 

 ihej are contained in the adult males; in plants the pollen may be 

 irradiated after it is shed. The effects of these irradiations may be 

 observed in a variety of ways. Number of eggs hatching, number 

 of larvae that develop to a given instar, and number of adult flies that 

 hatch are all measures that have been used in the past. For the 

 more direct effects of the radiant energy, gene mutation, chromosome 

 aberrations, etc. offer good quantitative measures of the changes pro- 

 duced. 



With pollen the viability may be determined by the growth of 

 the pollen tube on suitable artificial media or by the number of seeds 

 produced when the pollen is used for fertilization on a standard, ge- 

 netic type female. Chromosome and chromatid changes in the pollen 

 tube cells may measure the effect on the chromosomes. Seed and 

 plant characteristics present opportunities for gene mutation studies. 



In much of the earlier work multicellular seedlings were irradiated 

 and the effects of the irradiation determined by the growth of the 

 seedlings in the various treatments. The composite character of this 

 material made interpretation of the X-ray effects difficult. 



One of the most satisfactory methods of determining X-ray ef- 

 fects on vital cell stnictures is that of detecting so-called sex chromo- 

 some recessive lethals in Drosophila. This category includes several 

 radiation effects, such as gene mutations, small deficiencies in which 

 a vital gene or a series of genes is destroyed in the chromosome, trans- 

 location of a particular kind, etc. The detection of these chromosome 



