POLYMORPHISM 9 



reproduction. In the higher bacteria we may have what is in fact 

 a flower — terminal fructification by conidia. In this group of 

 vegetables we have the Beggiatoa, Leptothrix, Cladothrix, and, at the 

 top, the Streptothrix. It has been demonstrated that Streptothrix 

 actinomycotica and Streptothrix madurce are the organismal cause, 

 respectively, of Actinomycosis and Madura-foot, two diseases which 

 had hitherto been obscure. 



Polymorphism (or Pleomorphism). — This term is used to designate 

 an inconstancy of form or a tendency towards biological variation. 

 Vibrios may become spirilla, the ray fungus passes through a coccoid 

 and bacillary stage, and the diphtheria bacillus may either be long, 

 short, straight, or clubbed. This diversity of form appears to belong 

 to many species, and is transmitted from generation to generation ; 

 or the various forms may occur in succession, and represent different 

 stages in the life-history. In B. diphtherice, B. pestis, and B. tuber- 

 culosis and other forms, polymorphism undoubtedly occurs. It 

 is particularly marked in very old cultures of the last named. 

 The ordinary well-known bacillus may grow out into threads with 

 bulbous endings, granular filaments, "drumsticks," and diplococcal 

 forms. It is now known that amongst the causes of polymorphism 

 are certain adverse conditions of medium or other physical influences 

 (moisture, temperature, age, etc.), and thus some bacteria, especially 

 bacilli or vibrios, become altered in shape, losing their ordinary form. 

 On transferring such aberrant and abnormal forms to fresh medium 

 or favourable conditions, they are generally able to assume their 

 original morphology. Indeed the aberrant form is in all probability 

 only a stage in their life-history. Involution forms usually imply 

 degeneration. 



Biology of Bacteria 



Composition. — From what we have seen of the pabulum of 

 micro-organisms, we should conclude that in some form or other they 

 contain the elements nitrogen, carbon, and hydrogen. All three 

 substances are combined in the mycoprotein or protoplasm of which 

 the body of the microbe consists. This is generally homogeneous, 

 proteid material, and there is no sign of a nucleus. It possesses a 

 marked affinity, for aniline dyes, and by this means organisms are 

 stained for the microscope. Besides the variable quantity of 

 nitrogen present, mycoprotein may also contain various mineral 

 salts. The uniformity of the cell-protoplasm may be materially 

 affected by disintegration and segmentation due to degenerative 

 changes. Vacuoles, which it is necessary to differentiate from spores, 

 also may appear from a like cause. Vacuolation may also occur 

 as a result of a process of osmosis in salt solutions, the protoplasm 

 of the bacillus becoming contracted and disintegrated (plasmolysis). 



