308 THE MICROSCOPE AND ITS REVELATIONS. 



and thus, when they multiply to a sufficient extent, make their presence 

 apparent to the unaided eye, either as colored films on the sides of the 

 glass jars holding the solutions, or as (in the cases of blood-colored milk 

 and blue-green pus) imparting their color to the whole liquid. Liquids 

 in which any of these Schizomycetes^ are actively developing themselves, 

 usually bear on their surface a gelatinous scum, which is termed by Prof. 

 Colin (who first drew attention to it) the Zooglcea. This scum, when ex- 

 amined microscopically, is found to contain Scliizomycetes, sometimes of 

 several different kinds, and in different stages of development, often 

 mingled with true Infusoria, matted together into a mass; at the edges 

 of which they present themselves in a more separated condition, and 

 seem escaping to disperse themselves freely through the liquid beneath. 

 By Prof. Cohn. 1 who has made a special study of this group, it is con- 

 sidered to include a large number of generic and specific types, whose 

 distinctness is always preserved; but other observers, who have devoted 

 themselves to the more prolonged and complete study (by ' cultivation ') 

 of a small number of forms, seem to have made it clear that there is 

 at least in certain types a wide range of variation; so that when the 

 entire life-history of any one type shall be completely known, a number 

 of supposed species will be merged in it, either as transitory phases of 

 its existence, or as varieties resulting from differences in the media in 

 which they develop themselves. 2 There are, however, five well-marked 

 types, of each of which it will be desirable to give a separate account; 

 namely 1. Micrococcus; 2. Bacterium; 3. Bacillus; 4. Vibrio; and 

 5. Spirillum. 



304. The Micrococci are darkish or colored granules, so minute as not 

 to be measurable with certainty, and destitute of any power of move- 

 ment; which may occur either solitarily, or forming small groups or 

 beaded chains, such as would be produced by cell-division; but which 

 may also accumulate in irregular aggregations. The Monas prodigisa 

 of Ehrenberg, which is sometimes found imparting to the surface of 

 mouldy bread a blood-red tinge (attributed by the superstitious to a mi- 

 raculous exudation of blood), is regarded by Cohn as a Micrococcus. 

 There is considerable doubt whether any of these Micrococci are independ- 

 ent organisms; as it is certain that some of them are nothing else than 

 sporules of Bacteria or Bacilli (Plate xn., figs. 1-3). But as some of 

 them do not, under cultivation, develop themselves into any higher 

 form, continuing to multiply as isolated cells by binary subdivision, they 

 must for the present be ranked as distinct. 3 



305. Bacteria are minute oblong cells, which are usually seen at- 

 tached in pairs end to end (Fig. 193, A, c), but not unfrequently pre- 

 sent themselves singly (B, D), the pairs being produced by the self-divi- 

 sion of solitary cells. They are usually seen in ' vacillating ' movement, 

 produced by the action of their flagella, of which, in their paired state, 

 each cell bears one at its free extremity, whilst the solitary cells bear a 

 flagellum at each extremity. The formation of the second flagellum 

 seems to take place by the drawing-out of a filament of protoplasm be- 



1 " Beitrage zur Biologie der Pflanzen," Band i., Heft ii. (1872), and Heft iii. 

 (1875) ; Band, ii., Heft ii. (1870). 



2 See especially Prof. E. Ray Lankester's account of ' A Peach-colored Bacte- 

 rium,' in "Quart. Journ. Microsc. Science," Vol. xiii. (1873), p. 408; and Mr. J. 

 C. Ewart 'On the Life-history of Bacillus anthracis,' in Vol. xviii. (1878), p. 161, 

 of the same Journal. 



3 See Ewart in "Proceedings of Royal Society," June 20th, 1878. 



