MORPHOLOGY AND BIOLOGY OF BACTERIA. 21 



not end to end, but side by side in groups. Cocci thus 

 collected are designated staphylococci (from ffra<po^ y a 

 grape), because viewed microscopically the cocci resemble 

 bunches of grapes. (Fig. 7, /.) When 

 the bacteria appear joined in pairs, they ^ .... ... % 



are spoken of as diplococci (Fig. 7, U) or '-. '* ) 



diplobacilli. A number of bacteria divide / ^) 



in two or three successively vertical direc- 



tions. This mode of division has been Fi s- 6.-streptococci. 



observed only among cocci, and there thus 



result plate-cocci (tetragenus, tetrad arrangement) and the 



ball-of-twine-like packets known as sarcina. (Fig. 7, /.) 



In the process of cell-division an essential developmental 

 difference is observed between bacilli and cocci. The 

 bacilli become double their ordinary size before they are 

 ready for multiplication ; while the cocci break up, with- 

 out preliminary increase in size, into two hemispheres (or 

 in the case of tetrad arrangement into four quadrants, and 

 in the case of sarcina-formation into eight octants), and 

 spheric cocci then form from the products of division. 



f 



Fig. 7. Diagram illustrating the morphology of cocci: a, Coccus or micro- 

 coccus ; b, diplococcus ; c, d, streptococci ; <?, /, tetragenococci or merismopedia ; 

 g, h, modes of division of cocci ; z, sarcinae ; j, coccus with flagella ; k, staphylo- 

 cocci (McFarland). 



Vibrios also form pseudofilaments consisting of numerous 

 spiral cells, and, besides, though seldom, of long spirals 

 consisting each of a single cell. Division occurs among 

 these in much the same manner as among bacilli. 



For every bacterium there is a temperature at which it 

 thrives best (temperature-optimum), as well as a temperature- 

 limit above, and beyond which it can not survive (tempera- 

 ture-maximum), and one below, at which it will just live 

 (temperature -minimum). Among pathogenic bacteria the 



