DIMENSIONS OF BACTERIA. 35 



fore, formed the subject of exhaustive investigation with regard 

 to the internal construction of the bacterial cell. 



We may here briefly allude to a fission fungus which, although 

 unimportant from a practical, technical point of view, forms, 

 thanks to its large size, an especially 

 favourable object for the exhibition of 

 proportional dimensions, spore forma- 

 tion, &c., viz., the Bacillus megatherium, 

 found by DE BARY (I.) on cabbage 

 leaves. As the specific name would 

 imply, we have here an organism which 

 excels in size all other fission fungi, as 

 much as the prehistoric Megatherium 



, , . * , FIG. 5. Bacillus megatherium. 



surpassed his contemporary congeners. 



TVn fnvm is fnrfhpr illnitrntprl in w. Two individual rods. 



6. Two rods at the moment of 

 Fig. 5; the individuals ft, r, k, I, Will reproduction by fission. (After 



be described in Chapter vi. (treating of 



spore germination), so that only m and b 



need be considered at present. These are rods 2.5 \L wide and 10 ft 



in length, each of which would easily hold about ninety of the 



previously described cocci. 



It will be useful to remember that the wave-length of light 

 (corresponding to the spectral line D) radiating from the sodium 

 flame is about 0.6 p, i.e. about equal to the diameter of the above- 

 named lactic acid coccus. Bearing this in mind, the remark 

 already made, that the smallest of the bacteria are almost invisible, 

 becomes comprehensible. 



29. Mutability of Form. 



The question as to whether any given species of bacterium 

 assumes only one of the forms of growth already described, or has 

 the power of appearing in various shapes, is one of wide interest. 

 A reliable answer could, however, only be given when the necessary 

 appliances for studying the separate species in the form of pure 

 cultures, i.e. cultures practised upon an isolated single cell pro- 

 tected from subsequent contamination by other organisms, had 

 been invented. This possibility was first achieved early in the 

 " eighties," since which time the investigation of this question has 

 proceeded with energy, and the results have shown that the theory 

 of uniformity of growth (monomorphism) is untenable for bacteria, 

 and must give place to the theory of multiformity (pleomorphism). 



The remark made in a previous paragraph that the names 

 bacillus, coccus, &c., simply indicate certain forms of growth, 

 will now first become perfectly clear. When, in future pages, 

 one or other of the various bacteria is qualified by the generic 

 name Bacillus (for instance, Bacillus urece), it is not meant that the 



