8 Lectures on Bacteria. \^ i. 



tion of the observer only, is proved by the fact that they 

 appear in photographs. But in an overwhelming majority of 

 cases no cilia can be seen, though the Bacteria are capable of 

 independent movement and are examined with the best optical 

 aids after being killed and coloured. Where they are found, 

 they are as van Tieghem rightly says, not processes of the 

 protoplasmic body, but belong to the membrane, as is shown 

 by their behaviour with reagents, and must therefore be con- 

 sidered to be thread-like extensions of the soft gelatinous 

 membrane-layers. They have accordingly nothing in common 

 with the cilia of swarm-spores of the Algae, and cannot therefore 

 be regarded as organs of motion, since it was only from the 

 analogy of the cilia in the Algae that this function was inferred. 

 Such is the state of the case at least in the great majority of 

 species. Whether there are any exceptional cases must be deter- 

 mined by further investigation. It should be added, that among 

 lower organisms there are some comparatively large forms, 

 the Oscillatorieae, for example, the near relatives according to 

 our present knowledge of the Bacteria — a point to be further 

 considered below — which show similar movements, though no 

 cilia or other distinct organs of motion have been observed in 

 them. It follows that analogy does not require the discovery 

 of cilia in the Bacteria. 



Vegetating Bacterium-cells multiply by successive division, 

 each cell forming two daughter-cells. When a cell has reached 

 a certain size, a fine transverse line makes its appearance in it, 

 dividing the cell into two equal parts. This line is subse- 

 quently shown by its gelatinous swelling to be the commence- 

 ment of a cell-membrane. This agrees with the phenomena 

 observed in the divisions of larger plant-cells, and there is 

 nothing to prevent our assuming that the details of the process 

 of division, which the minuteness of the object makes it impos- 

 sible to observe directly, are the same in both cases. 



It must be acknowledged that the transverse wall which 

 appears as the cell divides is often so delicate as easily 



