10 BIOLOGY AND TECHNIQUE 



ing a volume of one cubic millimeter. Among the bacilli the range of 

 size is subject to even greater variations. Probably the smallest of 

 the common bacilli is the bacillus of influenza, which measures about 

 .5 fji in length by .2 /* in thickness. The limit o!' the optical possi- 

 bilities of the modern microscope is almost reached by some of the 

 known microorganisms, and there are some diseases which are caused 

 by organisms so small as to be invisible by any of our present 

 methods. In fact, the virus causing the peripneumonia of cattle has 

 been shown to pass through the pores of a Berkefeld filter, which 

 are impenetrable to the smallest of the known bacteria. 1 The coc- 

 coid or globoid bodies grown by Noguchi from poliomyelitis virus 

 are small enough to pass through such filters, but are still visible with 

 the highest lens magnifications. Whether or not these minute bodies 

 should be regarded as bacteria is questionable. It seems likely that 

 they represent an entirely different class of organisms. It is worth 



FIG. 1. TYPES OF BACTERIAL MORPHOLOGY. 



mentioning, also, that organisms like streptococci may show minute 

 forms, especially when grown under anaerobic conditions, which are 

 almost as small as the globoid bodies. It seems a general rule that 

 anaerobically grown cocci may assume smaller forms. 



MORPHOLOGY OF THE BACTERIAL CELL. When unstained, most 

 bacteria are transparent, colorless, and apparently homogeneous 

 bodies with a low refractive index. The cells themselves consist of 

 a mass of protoplasm, surrounded, in most instances, by a delicate 

 cell membrane. 



The presence of a nucleus 2 in bacterial cells, though denied by 

 the earlier writers, has been demonstrated beyond question by Zett- 

 now, Nakanishi, 3 and others. The original opinion of Zettnow was 

 that the entire bacterial body consisted of nuclear material inti- 



1 Nocard and Eoux, Ann. Past., 12, 1898. 



2 A. Fischer, Jahrbiicher f . wissen. Botanik, xxvii. 



3 Nakatnishi, Munch, med. Woch., vi, 1900. 



