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Thank you for your letter of the 9th. I am afraid I have no methods of the type you suggest. My own ideas are along the
lines of modifications of the "layer plate" technique, either by use of cellophane membranes or by allowing active
substances to diffuse upwards into the inoculated agar. With fast spreading moulds these techniques have manifest disadvantages,
and though in theory they sound satisfying enough I have not had time to experiment much with them. On the basis of my experience
I would suggest (for P. molstum or Asp. [...]) the growth of the plate of agar until organisms just appear. The removal either
by heat (a hot point on rod) of the grown colonies and then the raising of the plate by interposition of sterile stainless
steel gauge or fillerpaper to allow liquid containing the factor to be seen underneath, or alternatively the whole agar layer
could be bodily
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placed upon a prepared ager-growth substance gel.
I have, a published, used the filtration technique of Fries. This appears to depend on the power of germination of the organism
in the absence of the deferent factor. Usually the spores will be able to germinate on their reserves to a sufficient extent
to make them "particle size", considered from the point of view of filtration, comparable with that of the normal.
Again one would expect their heat- or alcohol-susceptibility to be much the same.
I have tried no mano-selection in repeated inoculation, via mass spore-suspension-molecula, in presence of the substance whose
deficient mutants are under study. This appears to me to have possibilities as a mode of bacterial study, but with moulds
the difficulties in distinguishing truly identical mutants from similar mutants (ie members of the same and different clones)
would under any study of mutation frequencies liable to error. However this might prove a useful way of obtaining large numbers
of mutants if the mass spore technique was accompanied by successive mass exposure to mutagen.
As my work is more concerned with the chemical synthetic mechanisms (a paper will shortly appear in Biochemical and Biophysica
Acta). I am engaged now upon quantitative work stemming from the original findings. So far these appear to be confirmed although
the picture may not be quite so simple as those represented. I will let you have the reprint if I manage to get any.
Thanking you for your very kind interest and hoping that my observations will be at least a little use to you.