46 MORPHOLOGY AND CULTURE OF MICROORGANISMS. 



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

 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 will 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. 26). 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 

 GO r3*ow 



FIG. 26. Division forms of micrococci. a, diplococcus, perfect form with flattened 

 opposed surface (gonococcus) , lanceolate form (pneumococcus) ; b, streptococcus; c, con- 

 secutive fission yielding 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 occupying 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 

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

 lococcij 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 bacilli, 

 one diameter is usually considerably shorter than the other, so that nature 

 almost invariably throws the new cell wall across the bacilli at right angles 

 to their long axis (Fig. 27). There is, therefore, only one arrangement or 

 cell grouping possible, and that is end to end, so that streptobacilli 

 are formed. When arranged in pairs, the designation is diplobacilli. 

 The length of the chains appears to depend not only upon the cohesion 

 of the bacilli but also upon the shape of the end; those which have square 



