4 GENERAL MORPHOLOGY AND BIOLOGY. 



which are usually chosen as good stains for the nuclei of 

 animal cells. Certain points have thus been determined. 

 The bacterial cell consists of a sharply contoured 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. Our knowledge of micro-chemistry 

 is, however, too scanty to justify any definite conclusion 

 being drawn on this point. 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 can 

 in some cases be demonstrated by overstaining a specimen 

 with a strong aniline dye, 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. 56). 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. Many of the higher bacteria possess a sheath 

 which, though probably a cellular excretion, has a much 

 more definite structure than is found among the lower forms. 

 It resists external influences, possesses elasticity, and serves 

 to bind the elements of the organism together. 



