THREE DECADES PROGRESS IN MICROBIOLOGY 



of microbiologists will find vast areas of their science still awaiting 

 cultivation. 



Until now I have only been dealing with observations made on 

 micro-organisms as they have been isolated from their natural habitat, 

 and the constancy of their properties during these observations has 

 been a postulate for the discussion of the results obtained. This means 

 that one of the most fundamental characters of every form of life, viz., 

 its variability, has been left out of consideration. 



Now it is a common experience of every microbiologist that not 

 rarely a continued study of some micro-organism leads to the conclu- 

 sion that variations have occurred. 



Fortunately in discussing the possibilities inhaerent in microbial 

 variations it is not necessary to have recourse to the still difficultly 

 interpretable results obtained with bacteria. There are other groups 

 of micro-organisms in which because of the occurrence of sexual 

 reproduction the nature of the variation phenomena is open to a 

 detailed analysis. 



Amongst the various studies made in this field the series of investig- 

 ations in recent years carried out by Beadle and his collaborators at 

 Stanford University on the biochemical genetics of the mould Neuro- 

 spora crassa may be deemed to be outstanding, and there are even sci- 

 entists who expect that this mould will in the course of time supersede 

 Drosophila from its central position in genetics. How this may be, the 

 said investigations are in any case apt to give a clear idea of the im- 

 portance which induced mutations may well obtain in the future devel- 

 opment of microbiology. It is for this reason that it seems worth while 

 to spare some moments for the discussion of some of the results ob- 

 tained. 



Neurospora crassa, commonly known as red bread mould, is heter- 

 othallic, that is, exists in two morphologically identical but physio- 

 logically different sexes. Each of these is haploid and by itself repro- 

 duces only vegetatively by mycelial growth or through the mediation 

 of asexual spores. If mycelia of the two sexes are grown together on a 

 suitable medium hyphal fusions accompanied by nuclear fusions occur 

 and out of the diploid zygotes ultimately asci containing 8 linearly 

 arranged ascospores are formed. Owing to the meiosis occurring in the 

 ascus preceding the ascospore formation the ascospores are again hap- 

 loid. If the parent strains differ in the alleles of a single gene, the eight 



387 



