38 BACTERIA. 



right angles to the long axis of the young organism, as has 

 already been described. It was at one time thought that in 

 the case of Bacillus subtilis division at first went on longitu- 

 dinally, but it has now been demonstrated that the general 

 rule is not departed from, the well-known appearance being 

 due to the fact that after rupture of the membrane the two 

 halves remaining attached by a kind of hinge are thrown 

 outwards, the ends of the vegetative spore protoplasm remain- 

 ing within these little cups, and the body growing rapidly 

 before division takes place ; a kind of loop is thus formed 

 which gradually becomes longer and longer, and the two 

 limbs which were supposed to be the result of a longitudinal 

 division are nothing more than the two ends of the same 

 rod. After a time transverse divisions may be seen in the 

 different parts of the loop, the ends escape from the cups 

 formed by the halves of the opened-up spore capsule, 

 the rods straighten out and assume the regular straight 

 form. There are other modifications of the same process 

 of development from the spore, but the above are the 

 essential or most important forms. 



Classification. 



Our present classification of Bacteria is based upon that given 

 by Cohn. He arranged these organisms into four groups, 

 taking as his characterising feature the form that was most 

 commonly assumed by each organism. His first group con- 

 sisted of rounded bacteria or cocci — the Sphsero-bacteria ; the 

 second group was made up of short rods, or cylinder-shaped 

 bacteria — Micro-bacteria ; longer rods or thread-like organ- 

 isms — the Desmo-bacteria — were placed in the third group ; 

 and to the fourth group were assigned screw-shaped or spiral 

 bacteria — the Spiro-bacteria. According to this author the 

 cocci consist of rounded or ellipsoid bodies, .5 to 2/i' in 

 diameter, the smaller ones being spoken of as Micrococci, 

 the larger as Mega- or Macro-cocci. They are found singly, 

 or may be grouped in pairs or in longer chains. The 

 micro-bacteria or short rods are more variable in their size, 

 measuring ifi in diameter and a little more than that in 

 length ; but when the length of an organism reaches more 



■ 1/4 = o.ooi mm. = taoooo g'h part of a metre = j^o^th part of an 

 inch. 



