HISTORIC DEVELOPMENT OF IMMUNOLOGY 25 



thought of as a true sphere. It may be elongated 

 or lancet-shaped as frequently seen in the diplo- 

 coccus of pneumonia; or biscuit-shaped, where 

 the cocci appear to be flattened against one an- 

 other, as in the diplococcus of gonorrhea. Those 

 forms which divide in one direction only and re- 

 main attached are found in pairs called diplo- 

 cocci, or in chains called streptococci. Those 

 which divide in any axis are found in irregular 

 grape-like bunches and are called staphylococci. 

 (2) The characteristic of the rod form, or ba- 

 cillus, is a straight axis with uniform thickness 

 throughout and flat ends. Sometimes the bacil- 

 lus has rounded ends. Some, as for example 

 the diphtheria bacillus, frequently are of un- 

 equal thickness at different portions. (3) The 

 spiral form, or spirillum, may be a true spiral 

 in shape or only a segment of a spiral. Here, 

 too, we have large and small, slender and thick 

 spirals. 



