THE PROBLEM OF MITOGENETIC HAYS 931 



sender; but this point has not been worked out in detail. This brings 

 us to the time of exposure. The time period must be determined for 

 each organism, set-up, sender, etc., separately. The time apparently 

 is very important, since an exposure which is too long may give negative 

 effects. Apparently the detectors have several maxima of response (289). 

 For this reason it is advisable to repeat each experiment several times 

 at different lengths of exposure since even the same detector does not 

 always respond to the same time of exposure. A detailed discussion 

 of the variation in response is given by Salkind (241). However, the 

 point of "mitogenetic depression" is not clear. Gurwitsch (108) in 

 his book (p. 13) reports that the mitogenetic effect with the same detector 

 should always give the same effect, that is, always an increase or always 

 a decrease — all positive or all negative effects. Workers in this field 

 report, especially with the liquid-yeast technique, that plus and minus 

 effects can appear side by side (Karpas, 174). In the work on bacteria 

 positive results reported are always plus effects. Salkind (241) dis- 

 tinguishes different types of mitogenetic depression, where he counted 

 both the change in percentage of buds and the increase in the total 

 number of yeast cells. He calls the increase in the amount of budding 

 "stimulation"; an increase in budding and in the number of mother cells 

 is also "stimulation"; a decrease in budding and an increase in the total 

 number of cells is "stimulated depression"; while a decrease in budding 

 and a decrease in total cells is "true depression." 



Gurwitsch reports that it is easier to modify the time of exposure, 

 and to diminish the total time of exposure by interrupting the exposure 

 systematically by employing a rotating disk which has several cut-outs. 

 It is thought that with a number of interrupted exposures the time 

 element can be more conveniently arranged. Investigators working 

 on detectors other than yeast have not reported very great success 

 with interrupted exposures, and in general the work done is insufficient 

 to justify detailed discussion at this time. 



It is known that chemical vapors may have a pronounced influence 

 on biological materials, either depressing or stimulating effects, and 

 all precautions against any possible influence of this type should be 

 taken. Exposures should be made only through quartz with high 

 transmission for the short ultra-violet radiation. Moreover, a piece 

 of quartz between sender and detector is not sufficient; if possible, the 

 detector should be closed vapor-tight from the sender in a container 

 with a quartz window (see Fig. 2). Have these precautions always 

 been taken? It would perhaps be unfair to generalize. 



But there are articles — too many to be ignored — in which mention 

 is made of the use of quartz between sender and detector in at least 

 one trial experiment, the remainder of the work being done wholly 

 without this precaution. No difference in effects was reported; so it is 



