Meter: 13 



The water hatch, of course, is the same throughout and nay be 

 represented as a straight line. The diffusate can be too strong giving 

 an inhibiting effect. The ijnportant thing is that there is a region on 

 the cui-ve through which a straight line can be dravm. That straight 

 line cuts through the line of the water hatch and this intersection is 

 used as a reference point. The dilution value at this point can be 

 conveniently expressed in logarithms, for example, a dilution of l/lOO 

 has a log of 2. V/e would then express the strength of the diffusate as 

 having a log activity value of 2, Use of this procedure does give us 

 a means of standardizing the diffusate. 



Suppose the biochemists are working on the active factor in a root 

 diffusate. They may extract it and produce what we call unknowns; there 

 may be several of them and we want to test their activity. The man who 

 is responsible for the assay has to ask the chemist about the level of 

 extraction, or how much it has been concentrated. That is, in making 

 up these tests the fractions have to be of such ? concentration or 

 dilution that the straight line portion of the curve will he found. 

 For example, if the unknowns are too concentrated they may give an 

 inhibiting effect and one couldn't get any measure of hatch. So you 

 try to set up about five dilutions, at least three of which you hope 

 will fall on the straight line portion of the curve. Then you set up 

 the batches in five-fold replications with not less than 100 cysts 

 per replica, as mentioned before. Often the batches of cysts are 

 weighed out, not counted, which is just about as accurate and much 

 quicker. You also set up a check with the standard root diffusate of 

 known log activity. From the results you can estimate the activity 

 of the fractions produced by the biochemist. And to conclude, I want 

 to add that there is not much use in taking the hatching results as of 

 only a week or two. The tests must be permitted to run to completion, 

 I have heard Dr, Fenwick tell chemists who have come with their series 

 of chemicals and ask for the results the next day, "You take a hen and 

 put some eggs under her. You just cannot get chickens in a day. You 

 simply have to wait for them to hatch out. It is the same with nema- 

 todes, you cannot get hatching in a day; you have to wait until the 

 test has gone to completion," 



