14 BACTERIOLOGICAL INTRODUCTION 



the resulting cells remaining coherent. Obviously 

 this group differs from the diplococci only in the co- 

 herence of the successive pairs of cells, and, indeed, 

 it is not rare to find short chains of four or six 

 members among the diplococci and pairs of cells 

 among the streptococci. The not unfrequent dumb- 

 bell-like arrangement in pairs of the cells comprising 

 a streptococcus betrays this method of growth, and 

 can be seen in several of the shorter chains in the 

 photograph. The variation in size of the individual 

 cells appears also. 



The above are the chief sub-groups formed by 

 uniaxial division of the spherical cell, but by a biaxial 

 division in planes at right angles to one another the 

 original cell gives rise to a group of four, and these 

 tetrads are grouped under the name. 



(d) Merismopedia. — The several varieties of the 

 organism Tetragenus belong to this group, and that 

 form which is frequently met with in cavities in 

 phthisis is shown in Fig. 5. Micrococcus tetragenus 

 is apparently harmless in man, but is pathogenic in 

 the mouse, and from the peritoneal fluid of such an 

 inoculated mouse the preparation is made. This 

 slide shows, in addition, the fact that a micro- 

 organism may be surrounded by a distinct capsule. 

 The capsule is a very inconstant feature, being 



