MORPHOLOGY AND PHYSIOLOGY OF BACTERIA. 13 



of the absorption of moisture and consequent swelling of the outer 

 layer of the cell-wall, a mucinous or gelatinous envelope develops 

 around them. This envelope may form a capsule, such as we meet 

 with in certain bacteria found in the rusty sputum of pneumonia, and 

 in Micrococcus tetragenus ; or it may occur as a continuous sheath 

 around a chain of bacteria, which by its disappearance sets free 

 the individual links. The capsule is soluble in water, and under 

 some circumstances is difficult to demonstrate. In the pneumo- 

 coccus of Friedlander the capsule disappears on cultivation, but 

 reappears in preparations made from an inoculated animal. In the 

 pleuritic fluid of a mouse these cocci are often found with a parti- 

 cularly well-marked capsule, and in other encapsuled cocci the extent 

 of the envelope has been observed to vary considerably in the same 

 species of bacterium. 



When this gelatinous material forms a matrix, in which 

 numbers of bacteria are congregated in an irregular mass, we have 

 what is termed a zoogloea. The zooglcean stage is a resting stage, 

 often preceded or followed by a motile stage. Thus bacteria may 

 be present in a solution in an active state, and after a time a scum 

 or pellicle forms on the surface of the liquid, which consists of 

 zooglcea. At the edges of the zooglcea, individuals may be seen to 

 again become motile, and after detaching themselves to swim off in 

 the surrounding fluid. 



The zoogloean stage may be observed sometimes in cultivations in 

 broth, and also in nutrient gelatine which has become liquefied. 

 The inoculated bacteria grow and multiply, and after a time a film 

 appears on the surface of the liquefied layer. In cultivations on 

 potato the appearances in this stage are varied, and sometimes 

 extremely characteristic. In the case of a bacillus which readily 

 develops on unsterilised potatoes, the zoogloea may spread over the 

 cut surface, forming a pellicle which can be raised en masse like 

 a delicate veil. Another bacillus forms a zooglcea, consisting of a 

 tenacious layer w^hich can be drawn out in long stringy threads. 

 In Ascococcus Billrothii the gelatinous envelope develops to such 

 an enormous extent that it forms the characteristic feature of the 

 secies. (Fig. 1.) 



Form. The individual cells vary in form, and may either 

 remain isolated or attached to each other. Hound cells and egg- 

 shaped cells are called cocci. The spherical form is the mast 

 common, but cocci are occasionally exclusively ovoid, as in Strepto- 

 coccus bombycis. The giant cocci of some species are spoken of as 

 j/iegacocci, to distinguish them from the ordinary cocci, or micrococci. 



