THE BACTEKIA 273 



in this species and in a very large group of allied 

 Bacteria, are endospores, one spore being produced in the 

 interior of each cell. At its first appearance the young 

 spore is of small size, appearing like a mere granule in 

 the protoplasm of the parent cell. As its growth pro- 

 ceeds it becomes elliptical in form, and increases suffi- 

 ciently in bulk for its walls to touch those of the 

 mother-cell. In the mean time it has completely used 

 up the surrounding protoplasm, and now lies within a 

 mere empty membrane. The endospore itself acquires a 

 comparatively thick cell- wall, and is extraordinarily 

 tenacious of life. These spores can bear being com- 

 pletely dried up without injury; they are little affected 

 by poisons, and survive a very high temperature, with- 

 standing even an hour's boiling in the case of the hay 

 Bacillus. Hence spore-forming Bacteria are extremely 

 difficult to extirpate, so that in order to make sure of 

 effectually " sterilising " any substance (i.e. destroying 

 any living things which it contains) it is often necessary 

 to expose it to a temperature considerably above the 

 boiling-point of water, or, if that be impracticable, at 

 least to continue boiling for some hours. 



The spores germinate when brought into a suitable 

 food-solution. The outer membrane splits across, and 

 the entire contents escape as an ordinary bacterial 

 cell, which at once begins to move about by means of 

 cilia (see Fig. 106, B). 



Fig. 107 shows very completely the stages in the 

 formation and germination of the spores in another 

 Bacillus, called B. megatherium, because for one of the 

 Bacteria it is quite a monster, though its cells are only 

 about 4Joth of a millimetre in diameter. This species 

 was originally found in boiled cabbages, and was after- 

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