DISTINCTIVE CHARACTERS OF THE BACTERIA. 171 



degree unpractical, and would retard for a long time, 

 and practically prevent, the further progress of our 

 knowledge with regard to the bacteria. 



In the present state of our knowledge it is best to em- Morphological 



, , -, . T . -i i characters of 



ploy both the morphological and the biological characters the four chief 



of the bacteria for their classification and distinction. - ron i )S - 



Four great divisions are in the first place made by 



selecting a single vegetative form or a definite and 



limited cycle of vegetative forms which are common 



to a large number of species of bacteria. To the first 



belong all those bacteria which only occur in the 



vegetative form, micrococcus. In the second we include 



the bacilli which occur in the form of rods, at times 



fclso in that of threads or spores. The third division - 



is composed of the spirilla, which are only met with 



in the form of spirals, or of fragments of spirals. 



The fourth includes those bacteria in whose cycle 



of development a variety of vegetative forms occur. 



The chief characteristics of these four divisions are the 



following : 



DIVISION I. MICROCOCCI. 



Spherical or egg-shaped cells multiplying by fission, Morphological 

 and always forming spherical cells ; they do not show the micro- 

 spontaneous movement. (The micrococci often exhibit, c< 

 as above mentioned, a trembling molecular movement, 

 and frequently also an alteration of situation due 

 to imperceptible currents of fluid. On closer observation 

 these can, however, be very easily distinguished from 

 true spontaneous movement.) Further, in the micro- 

 cocci endogenous spore formation does not occur; 

 we either observe no spore formation at all (in most 

 cases), or else so-called arthrospores are present and 

 are distinguished from the other cells by their size and 

 refracting character. In some micrococci the cells 

 are not accurately isodiametric, but the one diameter 

 exceeds the other to such an extent that an oval or 

 egg form results. Further, the spherical form is often 



