MORPHOLOGY, REPRODUCTION, ETC 13 



Zettnow, 12 who has carefully studied the structure of some of the 

 larger forms, takes the latter view, and regards the ' ' ectoplasmic " 

 zone as a part of the cell protoplasm devoid of nuclear material. 

 Zettnow's opinion is borne out by the greatly increased size of the 

 bacterial cells as seen by means of special stains. 



Many bacteria have been shown to possess a mucoid or gelatinous 

 envelope or capsule. According to Migula, 13 such an envelope is 

 present on all bacteria, though it is in only a few species that it is 

 sufficiently well developed and stable to be easily demonstrable and 

 of differential value. When stained, the capsule takes the ordinary 

 anilin dyes less deeply than does the bacterial cell body, and varies 

 greatly in thickness, ranging from a thin, just visible margin to 

 dimensions four or five times exceeding the actual size of the bac- 

 terial body itself. This structure is perfectly developed in a limited 

 number of bacteria only in which it then becomes an important aid 

 to identification. Most prominent among such bacteria are Diplo- 

 coccus pneumonias, Micrococcus tetragenus, the bacilli of the Fried- 

 lander group, and B. aerogenes capsulatus. The development of the 

 capsule seems to depend intimately upon the environment from 

 which the bacteria are taken. It is most easily demonstrable in 

 preparations of bacteria taken directly from animal tissues and 

 fluids, or from media containing animal serum or milk. If cultivated 

 for a prolonged period upon artificial media, many otherwise capsu- 

 lated microorganisms no longer show this characteristic structure. 



Capsules may be demonstrated on bacteria taken from artificial 

 media most successfully when albuminous substances, such as ascitic 

 fluid or blood serum, are present in the culture media, or when the 

 bacteria are smeared upon cover slip or slide in a drop of beef or 

 other serum. 14 Most observers believe that the capsule represents a 

 swelling of the ectoplasmic zone of bacteria. By others it is regarded 

 as an evidence of the formation of a mucoid intercellular substance, 

 some of which remains adherent to the individual bacteria when 

 removed from cultures. It is noticeable, indeed, that some of the 

 capsulated bacteria, especially Streptococcus mucosus and B. muco- 

 sus capsulatus, develop such slimy and gelatinous colonies that, when 

 these are touched with a platinum wire, mucoid threads and strings 



12 Zettnow, loc. cit. 



13 Migula, "Systeme d. Bakterien," 1, p. 56. 



14 Hiss, Jour. Exp. Med., vi, 1905. 



