286 BACTERIOLOGY. 



it may occur as a result of invasion of the tissues by 

 the organisra now to be described, viz., the micrococcus 

 tetragenus. 



This organism was discovered by Gaffky, and was 

 subsequently described by Koch in the account of his 

 experiments upon tuberculosis. It is often present in 

 the saliva of healthy individuals and is commonly 

 present in the sputum of tuberculous patients. Koch 

 found it very frequently in the pulmonary cavities of 

 phthisical patients. 



It, however, plays no part in the etiology of tuber- 

 culosis. 



It is a small round coccus of about 1 /i transverse 

 diameter. It is seen as single cells, joined in pairs 

 and in threes; but its most conspicuous grouping is in 

 fours, from which arrangement it takes its name. In 

 preparations made from cultures of this organism it 

 is not rare to find, here and there, single bodies which 

 are much larger than the other individuals in the field. 

 Close inspection reveals them to be cells in the initial 

 stage of division into twos and fours. A peculiarity 

 of this organism is that the cells are seen to be bound 

 together by a transparent gelatinous substance. 



When cultivated artificially it grows very slowly. 



Upon gelatin plates the colonies appear as round, 

 sharply circumscribed, punctiform masses which are 

 slightly elevated above the surface of the surrounding 

 medium. Under a low magnifying power they are seen 

 to be slightly granular and to present a more or less 

 glassy lustre. 



The colonies increase but little in size after the third 

 or fourth day. If cultivated as stab-cultures in gelatin, 

 there appears upon the surface at the point of iuocula- 



