268 XXI. BACTERIA YEAST. 



the bacterial masses often show definite colour, grey, yellowish 

 and even stronger colours. These colours are usually, if not 

 exclusively, confined to the membrane, and often serve for the 

 macroscopic distinction of the species. It is quite possible that 

 upon our vegetable disks such a strongly-coloured form may 

 appear ; for example Micrococcus prodigiosus looking like drops 

 of blood. 



Some bacteria are distinguished by the formation of an amy- 

 loid in their body, either in the stages preceding spore-formation 

 (as Bacillus Amylobacter), or independently of spore -formation 

 (as Bacterium Pastor ianum, and, occasionally, Bacillus buccalis) ; 

 and after addition of iodine solution either the entire mass, or to 

 the exclusion of definite cross-zones, stains blue to violet. 



Certain bacteria, known collectively as " sulphur bacteria," 

 contain sulphur granules of viscous consistence in their interior ; 

 others show iron-oxide deposits in their envelope, and are distin- 

 guished as " iron bacteria ". The iron-oxide deposits give to the 

 iron bacteria a brownish colour. 



In the pellicle which comes upon the surface of the leaf-infusion 

 we have also a form of Zooglcea (cf. Fig. 104 A) ; in it the cell- 

 rows are held together by jelly. This pellicle is traversed by deli- 

 cate, undulating, more or less parallel threads, formed of cocci or, 

 usually, of rodlets. Here also the segmentation of the cocci or 

 rodlets shows up specially clearly after the addition of iodine 

 solution. In a preparation taken from this culture we often see 

 the swarming stage of development ; but such can be obtained 

 with certainty in water in which peas have been soaked for one or 

 two days. We see then the bacteria in question in dancing move- 

 ment, hurrying about, now forwards, now backwards, in various 

 directions. Extremely fine cilia have been distinguished upon 

 swarming bacteria, and it is supposed that the movement is 

 brought about by their means (see Fig. 105). 



In exhausted nutrient substrata changes of form and contents 

 of the bacteria not infrequently come. These are considered to be 

 pathological or diseased conditions, prior to death, and the forms 

 showing then are distinguished as " involution forms " (Nagelii. 



If we now examine the pellicle of a leaf-infusion, such as above, 

 which has already stood for several days, we shall perhaps find 

 the rodlets or threads in spore-formation (Fig. 104 C). The con- 

 tents of the cells have collected at individual points, not more than 



