Liquid medium supplemented with 35% (w/v) PEG supported regeneration of the cell wall in protoplasts of Saccharomyces cerevisiae. The regeneration was followed by reversion to normal cells. Both these morphogenetic processes were similar to those described previously in gelatin media. The frequency of reversion was estimated to be 20 to 30%. The regeneration medium with PEG also induced protoplast fusion, When complementary auxotrophs were used for fusion only the fused products proliferated, giving first regenerated protoplasts and then hybrid cells.

Yeast cells in the stationary phase of growth are relatively resistant to snail enzyme digestion. This resistance was shown to decrease abruptly in the course of only 3-5 duplications, when stationary cells were allowed to grow in a fresh medium. The selective digestion of growing prototrophs from a mutagenized culture in minimal medium by snail enzyme was applied to increase the proportion of auxotrophs which remained relatively resistant.