4 GENERAL MORPHOLOGY AND BIOLOGY. 



determined. The bacterial cell consists of a sharply con- 

 toured mass of protoplasm which reacts to, especially basic, 

 aniline dyes like the nucleus of an animal cell. From this 

 fact it has been deduced that there is probably a close 

 relationship between the protoplasm of bacteria and the 

 chromatin of the nuclear protoplasm. To speak generally, 

 a healthy bacterium when stained presents the appearance 

 of a finely granular or almost homogeneous structure. The 

 protoplasm of the bacterial cell is surrounded by an envelope 

 which is probably of a gelatinous nature, and which can in 

 some cases be demonstrated by overstaining a specimen 

 with some of the aniline dyes, when it will appear as a halo 

 round the bacterium. This envelope may sometimes be 

 seen to be of considerable thickness. Its innermost layer 

 is probably of a denser consistence, and sharply contours 

 the contained protoplasm, giving the latter the appearance 

 of being surrounded by a membrane. It is only, however, 

 in some of the higher forms that a true membrane occurs. 

 Sometimes the outer margin of the envelope is sharply 

 defined, in which case the bacterium appears to have a 

 distinct capsule, and is known as a capsulated bacterium 

 (vide Fig. i, No. 4 ; and Fig. 50). The cohesion of bacteria 

 into masses depends largely on the character of the envelope. 

 If the latter is glutinous, then a large mass of the same 

 species may occur formed of individual bacteria embedded 

 in what appears to be a mass of jelly. When this occurs, 

 it is known as a zooglcea mass. On the other hand, if the 

 envelope has not this cohesive property the separation of 

 individuals may easily take place, especially in a fluid 

 medium in which they may float entirely free from one 

 another. The bacterial envelope does not, as in the case 

 of most vegetable cells, contain cellulose. 



Motility. As has been stated, many bacteria are motile. 

 Motility in a species is associated with the possession of 

 wavy thread-like appendages called flagella, which for their 

 demonstration require the application of special staining 

 methods (vide Fig. i, No. 1 2 ; and Fig. 78). They have been 

 shown to occur in many bacilli and spirilla, but only in two 



