IO 



INTRODUCTION : MORPHOLOGY 



clear space, sometimes narrower, sometimes broader than the cell it sur- 

 rounds (Fig. 7, c and d). It may be rendered more visible by special 

 methods of staining. It owes its origin to the swelling up of the outer 

 layers of the cell-wall by absorption of water, the inner layers being con- 

 stantly built up anew from within. This gelatinized membrane may become 

 more and more liquefied until large numbers of cells are buried up in a 

 common jelly-like mass, and it is in this way that the various forms of 

 zoogloea arise. The majority of bacteria have no such envelope at all, 

 or at most a thin invisible mucilaginous coat. The formation is often largely 

 dependent upon the nature of the food-stuffs available (Fig. 7, b and c). 

 Inasmuch as "many bacteria secrete large quantities of mucilage without 

 forming a definite envelope, the word ' capsule ' should be sparingly used 

 and restricted to clearly marked sheaths around the cell. Only in such 

 cases has it any classificatory value [Leuconostoc). Moreover, the clear 



spaces visible around bacteria in 

 dried preparations are frequently 

 artificial products. These are par- 

 ticularly prone to arise in fluids 

 like blood and lymph, which con- 

 tain albuminous substances, the 

 albumen being seemingly precipi- 

 tated upon the bacteria, around 

 which it forms a coating that 

 easily takes up stain. When dry- 

 ing takes place the cells, being the 

 last to give up their moisture, 

 shrink away from this envelope 

 and each one appears to be sur- 

 rounded by a capsule (Fig. 7, a). 

 That this is, however, nothing but 

 an artificial structure is shown by the fact that the same bacteria which 

 in the blood of an animal exhibit a capsule are, with some doubtful 

 exceptions, both in the tissues and in cultures, quite devoid of one. In 

 a mouse which has died of anthrax, the bacteria in the blood seem to have 

 a capsule (Fig. 7, a), whilst those in sections of the kidney are totally devoid 

 of one. That this is not due to a difference of the modes of growth in the 

 blood and in the tissues is shown by the fact that the naked anthrax bacilli 

 from an agar culture become provided with an apparent ' capsule ' if they are 

 rubbed up with a drop of blood from a healthy mouse and allowed to dry 

 upon a cover-glass. 



Among the trichobacteria [Cladothrix, Crenothrix) and some of the 

 cyanophyceae (Tolypothrix, Lyngbya), the outer layers of the cell-wall 

 undergo a hardening process which leads to the production of a firm sheath 



Fjg. 7. Capsules and mucous sheaths. a t Bac. an- 

 t/iracisv/iih so-called capsules in a dried streak-preparation 

 from the liver of a mouse ; for the nature of these and of 

 the capsules of other pathogenic bacteria, see p. 10. b-rd } 

 Leuconostoc mesenteroides (frog spawn fungus); b, on 

 non-saccharine media, without sheath ; c, with mucous 

 sheath pn medium containing sugar (o~c from Liesenherg 

 and Zopf) ; d y older zoogloea mass with chains of cells 

 (from van Tieghem). Magn. a 1500, 6 and c 1200, ^500. 



