926 



BIOLOGICAL EFFECTS OF RADIATION 



buds should be about 60 to 80 per thousand for the control, and 80 to 

 120 for the exposed if the effect is regarded as positive. Since an effect 

 is considered positive only when there is an increase of over 20 per cent 

 (see Table 2), lower percentages are regarded as an indication that 

 further experiments are needed. Gurwitsch (108) indicates in his book 

 that the slides are counted blindly; that is, the person who counts the 

 cells does not know whether he is counting the slide made from the 

 exposed or from the control block. However, from other publications it 

 is not obvious that this rule is followed. It would seem that one should 

 expect those working on the bud-counting method to count all slides 

 blindly, making this the real criterion of the reliability of the work. 



Table 2. — Mutoinduction of Yeast, Using Two Agar Blocks with a Culture 

 Surface of about 1 cm.", at 5 mm. Distance. Time of Exposure 15 

 MiN.; Fexation about 1 HR. after Exposure 

 {From Potozky, 213.) Each horizontal line represents one test 

 induced percentage — control percentage 



Percentage effect 



control percentage 



X 100 



It is, of course, obvious that this method of counting a large number of 

 yeast cells under the microscope and of continuously judging whether 

 one has a yeast bud or a mother cell is very tiresome and tedious. Those 

 who are working with this method • successfully report that they have 

 had very uniform results, much more uniform than those obtained with 

 other methods. In spite of the claim just mentioned, very few publica- 

 tions have appeared recently in which this method was used. It is 

 apparently being replaced by the liquid-yeast method. 



The culture medium for the liquid-yeast method is prepared in much 

 the same way as that for the solid-yeast method, omitting the agar. 

 To 10 cc. of the sterile beer wort referred to above 15 drops of a standard 

 yeast suspension are added. This culture is incubated for 16 hr. at 25°C. 

 The yeast suspension should then be in a vigorous stage of fermentation, 

 that is, there should be a constant stream of rising bubbles. The suspen- 

 sion is well mixed before using and the foam is permitted to settle. Two 

 capillary glass chambers, consisting of glass tubes, 1.5 mm. inside 



