Lecture VII. 53 



they are bright like specks of glass. It is easy to see that they 

 are not all the same shape. Vast numbers of them appear 

 as dots, and as they retain this outline no matter what their 

 movements are, we must conclude that they are excessively 

 minute spheres. These are called Cocci. Numbers appear 

 as short rods with a length of 3-6 times their diameter. Usu- 

 ally these rods are straight (Bacilli) but they occasionally are 

 curved (Comma bacilli). Then again we see occasionally 

 elongated forms worming their way among the rest with a 

 sinuous motion. These spirally twisted or corkscrew-shaped 

 organisms are called by bacteriologists Vibriones when they 

 consist of one or two turns ; when they are longer, Spirilla or 

 Spirochaetes. When straight bacilli cohere end to end they 

 give rise to a growth-form called Leptothrix. All these forms 



V 



i 



FIG. 10. Various forms of bacteria, a, staphylococcus ; b, diplococcus; c, 

 streptococcus ; d, sarcina ; e, planococcus ; /, bacillus single cilium 

 (monotrichous) ; g, bacillus with two tufts, h, with one tuft of cilia 

 g and h are called lophotrichous forms ; i and / peritrichous bacilli ; k, 

 filamentous bacilli ; m, comma bacilli ; n t vibriones ; o, spirochaetes. 



are exceedingly minute, as may easily be realised if yeast cells 

 are introduced into the fluid containing the bacteria. Some 

 of the longest cells may attain a length equal to the yeast cell 

 but their diameter is never more than one-third that of the 

 yeast. In measurements of bacteria it is customary to use 

 the micron as the unit, i micron (^) = o-ooi mm. Thus the 

 longest bacillus would be IO/A long, with a diameter of i to 

 3/x,. The cocci are usually about i/x in diameter. A few are 

 even credited with a diameter of only o'i5/x. Sometimes the 

 elongated kinds cohere together and form comparatively long 

 rods or threads. While we may thus gain some idea of their 

 size relative to other small organisms it is by no means easy 

 to visualise their absolute minuteness. Calculation shows that 

 17,000 million bacteria could fit in a cubic millimetre, while a 

 single drop of liquid culture might contain 10,000 millions. In 



