BACTERIA 85 



organism possessing them. They are of sufficient definiteness and 

 constancy to be used by the systematists in characterizing large groups. 

 Cell Aggregates among the Micrococci. — ^The grouping of 

 micrococci depends upon the plane of division and also upon the cohe- 

 sion of the cells. Since it is quite as economical for the micrococcus to 

 divide in one direction as another, it is possible for a number of difEerent 

 cell groupings to occur. Whatever the direction of the dividing walls, 

 it is usually quite constant; if a particular species of micrococci has its 

 planes of division parallel, there wiU be formed chains of micrococci. 

 In some cases the cohesion is slight and only two cells remain attached 

 to each other, forming what are ordinarily known as diplococci. There 

 is a considerable number of very well-known bacteria that are diplo- 

 cocci (Fig. 64). If the cohesion is stronger, we have chains of micro- 

 cocci or rosaries formed which are known as streptococci. Well-known 

 and very important bacteria are grouped in this way. In other micro- 

 cocci the cell wall is not formed continuously in parallel planes but in 



O 



a 



Fig. 64. — Division forms of micrococci, a, Diplococcus, perfect form with 

 flattened opposed surface {gonococcus), lanceolate form {pneumococcus); b, strepto- 

 coccus; c, consecutive fission jdelding a tetrad; d, sarcina form resulting from division 

 of tetrad c; e, staphylococcus. {After Novy.) , 



planes which alternate at right angles to each other. In this way cell 

 aggregates occupjdng two dimensions of space are formed. These are 

 known as tetracocci, or merismopedia. Still again, the planes of division 

 may proceed at right angles to each other in three dimensions of space. 

 In this case packets are formed which are known as packet cocci, or 

 sarcincB.' Another group of the micrococci occurs, known as the staphy- 

 lococci, so called because they are arranged in irregular bunches, like a 

 bunch of grapes. This arrangement may be due to the fact that these 

 micrococci divide in many different planes, or because during the course 

 of their growth their arrangement is changed. 



Cell Aggregates among the Bacilli. — In the case of the bacilU, 

 one diameter is usually considerably shorter thaji the other, so that 

 nature almost invariably throws the new cell wall across the bacilli 



