DEVELOPMENT AND COMPOSITION OF BACTERIA 9 



and the spiral. The rods are known as bacilli 

 (sing, bacillus), the spheres as cocci (sing, coccus), 

 and the spirals as spirilla (sing, spirillum). More 

 baccilli are known than any other form, and cocci 

 are much more numerous than spirilla. 



Long-continued growth in artificial media some- 

 times causes abnormal forms of growth, just as 

 unnatural surroundings and conditions produce 

 deformities in the higher forms of life. 



Structure. The internal structure of bacteria 

 is exceedingly simple : there appears to be a mem- 

 branous covering, including the protoplasm (ger- 

 minal matter), which is not always clearly discern- 

 ible, but in some species (micrococcus of pneumonia) 

 may be seen as a sharply defined capsule inclosing 

 a clear zone. 



The nature of the cell-substance is still imper- 

 fectly understood; the question of the existence 

 of a nucleus and the significance of the internal 

 structure are still disputed points which present one 

 of the exceeding difficulties of which Mliller wrote 

 in 1786. 



Motility. The power of motion in certain species 

 of bacteria is due to hair-like appendages, or flagella, 

 which by a lashing movement enable them to move 

 through fluids. The flagella vary in their position 



