198 BACTERIOLOGY. 



but be postponed until between the fourth and the eighth 

 day, it may occur as a result of invasion of the tissues 

 by the organism now to be described, viz., the micro- 

 coccus tetragon us. 



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 lung cavities of 

 phthisical patients. It, however, plays no part in the 

 etiology of tuberculosis. 



It is a small round coccus of about 1 /^ 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 prepa- 

 rations 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 in- 

 spection reveals these bodies 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 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 inocula- 



