122 



BACTERIOLOGY. 



readily develops on unsterilised potatoes, the zoo- 

 gloea may spread over the cut surface, forming a 

 pellicle which can be raised en masse like a delicate 

 veil. Another bacillus forms a zoogloea, consist- 

 ing of a tenacious layer which can be drawn out 

 in long stringy threads. In Ascococcus Billrothii the 

 gelatinous envelope develops to such an enormous 

 extent that it forms the characteristic feature of the 

 species (Fig. 46). 



FIG. 46. Ascococcus BILLROTHII, X 65. [After Cohn.] 



Form. The individual cells vary in form, and 

 may either remain isolated or attached to each 

 other. Round cells and egg-shaped cells are called 

 cocci. The spherical form is the most common, 

 but cocci are occasionally exclusively ovoid, as in 

 Streptococcus bombycis. The giant cocci of some 

 species are spoken of as megacocci^ to distinguish 

 them from the ordinary cocci, such as micrococci. 

 The fission by which the cocci increase may take 

 place in one direction only, and if the two resulting 



