C. DOBKLL 341 



belonging to the same race are brought side by side on the same 

 culture medium, they are seen to fuse as soon as their edges come 

 in contact. In the case of different races, however, no fusion occurs 

 between the contiguous edges under such conditions. Wolf endeavoured 

 by various means to obtain from a pure race (in which the swarms 

 readily fused) physiologically different races which would not coalesce 

 when brought in contact. He easily succeeded in doing so in races 

 belonging to two different species — Myxococcus rubescens and M. 

 virescens. He found that the same race could produce swarms which 

 would not fuse, if they were cultivated for a sufficiently long time. 

 The change occurred even when the organisms were grown on the 

 same medium and under the same conditions {M. rubesceiis). But the 

 change took place more rapidly if the swarms were cultivated at 

 different temperatures, on different media, or on media containing 

 various salts (KjCrjOy, KNO3, etc.). The peculiar feature of the 

 change was that it was irreversible. When once the swarms had 

 lost the power of fusing, no amount of further cultivation on normal 

 media would induce them to revert to their original condition. It 

 seems obvious, therefore, that under certain ill-defined conditions, a 

 permanent physiological change may occur. But it is by no means 

 obvious what the nature of the change is — its only manifestation 

 being an inability to fuse, which might be due to several different 

 causes. It seems impossible at present to draw any further conclusion 

 from these observations. 



A place must here be given to some statements just made by 

 Baerthlein (1912 a), as they concern the colour mutations of B. 

 prodigiosus and Staph, pyogenes. This worker states that by merely 

 plating out on agar old cultures (in agar or broth) of B. prodigiosus, 

 he can obtain numerous mutated colonies. These may be dai-k red, 

 pink, white, white with red spots, red with white sectors, etc. The 

 individual organisms in these different colonies also differ structurally 

 from one another. These mutated races are said to breed true. They 

 preserve all their characteristics when cultivated further on various 

 media or after passage through animals. "Atavistic phenomena in 

 the form of reversions" occur, however, if the mutated colonies are 

 left for a long time in the same medium, and then transferred to fresh 

 media. Similar changes are said to occur in the case of B. pyocyaneus. 

 Baerthlein's brief statements are not easy to reconcile with Wolf's careful 

 work on the same organism. It seems possible that he has not been 

 dealing with pure lines. 



