BACK-MUTATION AND PROGRESSIVE MUTATION 17-3 



even distribution of cells, A 5 per cent agar medium containing 

 4 per cent galactose cooled to 39^ was poured over the inoculated 

 surface. These test plates were incubated at 28*^0. for at least 

 48 hours. 





Fig. 17-2 Field of a Test Plate in which Colonies Grow 

 Embedded Between Two Layers of Agar. Positives (gas-producers) 

 are easily distinguished by the typical star— shaped crack in 

 the agar surrounding them. 



Colonies growing on a plate of this tj^e either produced a suf- 

 ficient amount of gas to make a star-shaped crack in the agar or 

 grew to make a flat colony lying horizontally between the two layers 

 of agar (fig. 17-2). Counts of ten test plates, each containing about 

 600 colonies, revealed that an average of from 2 to 15 per cent of 

 the colonies produced cracks. This warrants the conclusion that 

 from 2 to 15 per cent of the so-called nonfermenter haplophase cul- 

 tures are actually capable of fermenting galactose. When cells from 

 a fermenter colony of Db23B which had produced cracks on galac- 

 tose were similarly tested, 75 to 99 per cent of the cells produced 

 cracks. 



When the diploid culture (Lk), grown on glucose, was similarly 

 tested on galactose plates 100 per cent of the colonies produced the 

 star -shaped cracks in the agar. 



