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 fi in 

 length by .2 /* in thickness. The limit of the optical possibilities of the 

 modern microscope is almost reached by some of the known micro- 

 organisms, and it is not at all out of question that some of the diseases, 

 for which, up to the present time, no specific microorganisms have 



FIG. 1. TYPES OF BACTERIAL MORPHOLOGY. 



been found, may be caused by bacteria 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 



MORPHOLOGY OF THE BACTERIAL CELL 



When unstained, most bacteria are transparent, colorless, and ap- 

 parently homogeneous bodies with a low refractive index. The cells 

 themselves consist of a mass of protoplasm, surrounded, in most in- 

 stances, 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 Zettnow, 

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

 entire bacterial body consisted of nuclear material intimately inter- 

 mingled with the cytoplasm. The opinion now held by most observers 



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



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



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



