MORPHOLOGY OF ACETIC ACID BACTERIA. 387 



swelling and mutual fusion of the external layers of the cell 

 membranes, in which the cells then become embedded. In ordi- 

 nary (unstained) preparations the presence of this envelope is 

 only deducible from the mutual cohesion of the cells ; it can, 

 however, be rendered visible by suitable treating and staining, e.g. 

 by Loeffler's method. Fig. 8 (p. 40) was drawn from a preparation 

 of this kind. 



The behaviour of the mucinous envelopes of these three 

 species towards iodine solution (iodine in water or alcohol or 

 potassium iodide) is worthy of notice ; those of B. Pasteurianum 



FIG. 8 1. Bacterium aceti. 



Cells from a freshly formed skin on 

 " doppel-bier. " 



Magn. 1000. (After Hansen.) 



FIG. 82. Bacterium Pasteurianum. 



Cells from a freshly formed skin 

 grown at 34 C. on "doppel- 

 Ibier." 



FIG. 83. Bacterium Kutzingianum. Magn. 1000. (After Hansen). 



Cells from a freshly formed skin grown 

 at 34 C. on " doppel-bier." 



Magn. 1000. (After Hansen.) 



and B. Kutzingianum are thereby stained blue, whilst that of B. 

 aceti remains unaltered. The point must be emphasised that it is 

 the mucinous envelopes and not the true cell membranes that are 

 stained in this manner. The cell plasma is in all three cases 

 coloured yellow, consequently the preparations all exhibit yellow 

 cells after the iodine treatment, these cells being embedded in the 

 case of B. aceti in a colourless matrix. In B. Pasteurianum and 

 B. Kutzingianum this latter is blue, and to the unaided eye 

 the appearance of the whole varies from green to bluish- green, 

 according to the proportion of the matrix. It was this difference 

 in the behaviour of the mucinous envelopes which, however, is 

 noticeable only in young and vigorous pellicles that first directed 



