6 Lectures on Bacteria. [§ i. 



microscope as a delicate line drawn round the free surface, and 

 forming the boundary between contiguous cells. It may even be 

 seen distinct from the protoplasm in the larger forms by the aid 

 of reagents which strongly contract the protoplasm and colour it 

 at the same time without affecting the membrane, for instance 

 alcoholic solution of iodine (see Fig. i, /). It is plainly 

 shown also in the formation of spores which will be described 

 in Lecture III. This membrane, which lies close upon 

 the protoplasm, is in certain forms at least, the species of 

 Beggiatoa and Spirochaete for example, highly extensible and 

 elastic, for it is seen to follow the curves often made by the 

 elongated organism, and the protoplasm can alone be the active 

 agent in producing these. But the membrane which thus directly 

 covers the protoplasm is certainly in all cases only the innermost 

 firmer layer of a gelatinous envelope surrounding the proto- 

 plasmic body. This may be seen directly in not a few forms 

 if observed attentively under the microscope, when the cells 

 or small aggregations of cells lie isolated in a fluid. Large 

 masses of Bacteria are always more or less gelatinous or slimy 

 when in a suflSciently moist condition. When the cells are 

 dividing, the outer layers of the membrane may sometimes be 

 seen to swell up in succession. Hence, speaking generally, we 

 may say that the cells of Bacteria have gelatinous membranes, 

 with a thin and comparatively firm inner layer. The consistence 

 of the mucilage and its capability of swelling in fluids differ 

 in different cases, changing gradually, but this point will be 

 considered again presently at greater length. 



The possession of gelatinous membranes of this kind is com- 

 mon to the Bacteria and to various other organisms of the lower 

 sort, of which Nostocaceae and some Sprouting and Filamentous 

 Fungi may be quoted as examples. In Bacteria, as in the latter 

 plants, the gelatinous membrane has been shown in a number 

 of forms which have been examined to consist of a carbohydrate 

 closely related to cellulose ; this is specially the case in the Bac- 

 terium of mother of vinegar and in Leuconostoc, the frog-spawn- 



