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A MANUAL OF THE ASPERGILLI 



drastically altered conditions of growth), and hence can properly be termed 

 induced mutations, a number of well authenticated cases of natural muta- 

 tion are known. In 1939 Edward Yuill described as .4. fumigatus var. 

 helvola a buff-colored mutant of this species isolated by him in 1937 (fig. 



Fig. 17. Natural mutations. A, Portion of a colony of Aspergillus niger in which 

 a tan spored mutation appeared as a V-sector : Ai, typical black head of parent strain; 

 A 2 , tan head of naturally occurring mutation; X 25 approximately. B and C , Typical 

 strain of Aspergillus fumigatus and a naturally occurring mutation discovered and 

 described by Edward Yuill as Aspergillus fumigatus mut. helvola. The mutations 

 in both species have proved completely stable in continued culture. 



17 C). In the same report a white-spored mutant of A. nidulans, isolated 

 in 1937, was described as A. nidulans mut. alba. In both cases the mutants 

 developed as natural phenomena without the application of any artificial 

 stimuli — in the former case as a single head, in the later case as a group 

 of heads, and in both cases from wholly typical strains (fig. 17). 



