THE PROBLEM OF MI TOGENETIC RA YS 927 



diameter, are filled at the same time from the same pipetteful of suspen- 

 sion. The one tube is exposed to the sender at a distance of 5 to 10 mm. 

 for 5 to 10 min.; the other tube serves as a control. After exposure 

 0.3 CO. is removed from the detector and 0.3 cc. from the control chamber 

 and each is added to 1 cc. of wort. This is permitted to stand for 4 hr. 

 at 25°C. and then for 12 hr. at 21°C. Next, the yeast is killed with 

 0.5 cc. of 20 per cent sulfuric acid, after which the preparation is permitted 

 to stand for 10 min. and then diluted with 1 cc. of distilled water. 



There are at present several methods for determining the amount of 

 yeast present in the suspension. The simplest way to determine the 

 number of yeast cells is the one described by Potozky and Salkind (213). 

 Here all cells are counted in the Thomas-Zeiss blood-counting chamber. 

 Not less than 10 fields are counted, each with about 130 cells. One 

 precaution is suggested for this method, that is, that the yeast should 

 remain in the incubator for not more than 3 to 4 hr. and that then 

 it should be killed immediately on removal. The method is supposed to 

 give uniform results and to show a positive growth stimulation of about 30 

 per cent. However, this particular method is not generally used. 



A modification of this method has been used occasionally by Baron 

 (14, 241). Here the number of mother cells and the percentage of buds 

 are determined as in the solid-yeast method, but the Zeiss blood-counting 

 chambers are employed. This method gives two sets of data, one on the 

 increase of total yeast and one on the change in percentage of buds. It is 

 employed in certain detailed studies of yeast growth as influenced by 

 mitogenetic rays. 



The mycetocrit method described by Brainess is used at present by 

 most investigators who use yeast as the detector for mitogenetic rays. 

 Mycetocrits are haematocrits modified so that they are prepared without 

 glass beads. Each mycetocrit has, besides its number, two other mark- 

 ings; one at about 45 mm. from the tip gives the height of the uniform 

 capillary, the other mark near the top indicates the height to which 1 cc. 

 fills the mycetocrit. The mycetocrits are always prepared in pairs, that 

 is, from the same piece of capillary tubing, the capillary mark and the 

 1 cc. mark coming to the same height in both tubes. It should be possible 

 to use both tubes interchangeably. The tubes are tested before using 

 with mercury and should not differ more than 2 per cent. 



The yeast suspension in the test tube is well mixed by a uniform 

 standardized procedure; the material sucked into the mycetocrit, the 

 lower end sealed with cement, and the detector as well as the control 

 mycetocrit centrifuged at the same time for 10 min. at about 3000 r.p.m. 

 In zero experiments, that is, in experiments in which no exposure was 

 made, the heights of the yeast columns should not differ more than 5 per 

 cent. A difference of more than 20 per cent in the exposed over the 

 control is looked upon as a positive effect. 



