12 Lectures on Bacteria. [^ ii. 



Thinness of the gelatinous membranes and a high degree of 

 capacity for swelling reaching to deliquescence will cause the 

 separation of cells or of the simplest cell-unions from one 

 another when growing in a fluid. Thick cell-membranes, and 

 a narrowly limited capacity for swelling in the gelatinous sub- 

 stance, will keep the cells united together in compact gelatinous 

 masses in the same fluid. These, which are the extreme con- 

 ditions, are actually found in nature, and all kinds of intermediate 

 states also occur. The firmer gelatinous masses (see Fig. 3) 

 are called by the old name Palmella, or by the more recent 

 name now commonly used, Zoogloea. The less sharply defined 

 Zoogloeae, as they may be shortly described, may naturally be 

 termed swarms. It depends on their specific gravity whether a 

 Zoogloea or a swarm will float on the surface or sink to the 

 bottom of the same fluid, while their general outline and the 

 grouping of the separate aggregates which compose them will 

 be fashioned in accordance with their further specific qualities. 



To illustrate this point in passing by a few examples, — 

 let us take three flasks containing a similar 8-10 per cent, 

 solution of grape-sugar and extract of meat in water. In one 

 flask the fluid is pretty uniformly clouded with the short motile 

 rods of Bacillus Amylobacter. In the second the surface of 

 the slightly clouded fluid is covered with a thick wrinkled scum, 

 dry on the upper surface, which is formed by Bacillus subtilis, 

 the so-called hay-bacillus. In the third the filaments of Bacillus 

 Anthracis, the Bacillus of anthrax which in other respects re- 

 sembles B. subtilis, form a flocculent deposit at the bottom of 

 the clear fluid. We can scarcely call this deposit by the name 

 of Zoogloea, we may perhaps call it a swarm. The hay- 

 bacillus-scum is properly a Zoogloea with a special characteristic 

 form. Formations more or less like it are found often enough 

 in fluids containing decomposable organic bodies. Highly 

 characteristic Zoogloeae developed in a fluid are the frog-spawn- 

 Bacterium of sugar-factories and the Bacterium of kefir. The 

 former is a round-celled organism, Leuconostoc, with a thick 



