47 



uniform. In cultures on suitable media the same form is usually 

 observed although there is on the whole rather more poly- 

 morphism, particularly as regards the length of the individual 

 elements. It is far from rare however to find divergent forms 

 which may have a very different appearance: first of all 

 thread-forms often of considerable length (20—30 ^ or more). 

 They may be of the same thickness as the short forms or rather 

 thicker. Very frequently a few threads or long rods are seen 

 in an otherwise typical preparation. Next, forms like Pfeif- 

 fer's „pseudo-influenza bacillus" are met with, that is to say 

 rods which are longer and thicker than the typical form, and 

 often curved. Lastly a very variegated picture may l>e seen, 

 — large, round, oval, pear, or spindle formations, rods furnis- 

 hed with large or small protuberances or irregular swellings, 

 and sometimes branchings. The marked polymorphic forms are 

 comparable with the involution forms of the plague bacillus. 

 Wade & Manalang have described cultures which consisted ex- 

 clusively of long threads with rounded bodies of various sizes, 

 sometimes situated at the end and sometimes at the side, in 

 the latter case either on a small branch or directly on the 

 flank of the thread. 



There may be pronounced individual differences in the 

 tendency of the various strains to produce such forms but 

 an actual classification on these grounds cannot be made partly 

 because there are all possible transitions, and partly because 

 the same strain may often vary very considerably in such a 

 way that at one time the typical picture may be found, at 

 another a more or less atypical picture in different genera- 

 tions of the same strain, and also that the different forms 

 may be seen side by side in the same preparation. 



The irregular forms appear principally in old cultures and 

 on poor media (ascitic agar, Cantani's (2) bile agar, Ficht- 

 ner's sputum agar, blood agar containing citrate or other salts 

 in greater concentration than normal, too acid or too alkaline 

 media). Preuss found them especially at the end of the pan- 

 demic. 



But also under apparently unaltered conditions of growth 

 the same strain may at one time grow typically and at another, 

 in a very irregular manner. 



Sometimes the first culture on an artificial medium is 



