BACTERIA 79 



Involution Forms. * — The f orm^ of bacteria are quite constant under 

 normal conditions, but very frequently they show abnormal or bizarre 

 shapes. These are known as involiftion forms (Fig. 59). It is some- 

 times suggested that these involution forms represent another stage in 

 the developmental history of the organism, and upon this supposition 

 certain bacteria which very regularly show these involution forms have 

 been classified as belonging to a different suborder from that in which 

 the lower bacteria are placed. The ordinary view of the involution 

 forms is, however, that they are degeneration forms, that they cor- 

 respond, in other words, to the halt and maimed in society and are to 

 be accounted for by the fact that they are deformed by their own by- 

 products. In fact, it is quite probable that they are autogenic. In- 

 volution forms are very Ukely to occur in artificial culture and are much 

 more common with some species than with others. (See page 98.) 



Size* 



The bacteria were formerly spoken of as the smallest of Hving things, 

 but since the recognition of the ultramicroscopic organisms it is neces- 

 sary to be somewhat more specific in characterizing their dimensions. 

 The unit of measiurement in, microscopy is the micron (/i), or micro- 

 millimeter. This is .001 mm. or approximately 1/25000 of an inch. 

 Applying this unit to the bacteria we find that the micrococci and the 

 short diameter of the bacilli and spirilla average about i/u. The micro- 

 cocci vary in diameter from a small fraction of a micron to three or four 

 microns in diameter. The bacilli are sometimes very small, as the 

 influenza bacterium with a width of 0.2/i and a length of o.s/x, and 

 sometimes very large as, for example, the Bad. anthracis with a width 

 of i.2/i and a length of 5.20/^. The spirilla average about i.o/< in 

 diameter but may be as long as 30/1-40/i. 



Motility* 



When bacteria are viewed under the microscope in a living condition 

 many of them are seen to move. This movement may be one of two 

 kinds. In some cases it is progressive, the individuals move about from 

 one part of the field of the microscope to another and change their rela- 



•Prepared by W. D. Frost. 



