32 BACTERIA. 



inch cubes. This mode of division was first demonstrated by 

 Goodsir in the Sarcina ventriculi. 



We have already seen that these minute organisms are 

 endowed with great power of movement and locomotion, 

 a fact on which stress has been laid in connection with 

 their rapid diffusion through fluids ; but an equally im- 

 portant factor in the bringing about of this rapid diffusion 

 is their extreme prolificness. If they can obtain sufficient 

 food, and if the food is of exactly the right nature, the rate 

 at which bacteria grow is marvellous. From actual experi- 

 ment it has been found that if in a cubic centimetre of 

 any specimen of water, we find, say, a couple of hundred 

 organisms ; on standing for twenty-four hours the number 

 will have risen to about S,ooo per c. c. ; at the end of another 

 twenty-four hours, to 20,000 ; and on the fourth day they 

 are uncountable. Cohn calculated that a single germ could 

 produce, by simple fission, as above described, two of its kind 

 in one hour ; in the second hour these would be multiplied to 

 four, and in three day they would, if their surroundings were 

 ideally favourable, form a mass which can scarcely be reckoned 

 in numbers — or if reckoned, could scarcely be imagined — 

 4,772 billions. If we reduce this number to weight, we find 

 that the mass arising from this single germ would, in three 

 days, weigh no less than 7,500 tons. Fortunately for us, they 

 can seldom get food enough to carry on this appalling rate 

 of development, and a great number die both for want of 

 food, and because of the presence of other conditions un- 

 favourable to their existence. Vegetative multiplication 

 only takes place when the conditions are extremely favour- 

 able to the growth of the organism. If nutrition is in- 

 terfered with in any way, or if the removal of excretory 

 products is obstructed, or if there be a large amount of 

 oxygen present marked changes may at once be observed in 

 the appearance of the protoplasm of the micro-organism. It 

 becomes granular, then a small bright point appears in each 

 cell ; this point gradually increases in size until its diameter 

 may be greater than that of the original organism. This 

 large, clear, rounded, ovoid, or rod-shaped node is known as a 

 spore, or resting-spore. It is really the seed or egg formed 

 by the bacterium, by which the species may be continued 

 although the parent should perish. The shape varies slightly 

 in different species, but in every case it has a dark limiting out- 



