ESSENTIALS OF MILK BACTERIOLOGY -335 



carapace of the lobster, or the outer wing-cases of beetles. Vege- 

 table protoplasm, on the other hand, is contained usually in a cell 

 wall of cellulose, while animal protoplasm has no wall distinct 

 in composition from its own substance. The protoplasm of the 

 bacterium, contained in its surrounding wall, forms what is called a 

 cell, and since each such cell, amongst the bacteria, is complete in 

 itself and capable of independent existence, feeding, growing, multi- 

 plying, excreting, dying, the bacteria are spoken of as uni-cellular 

 organisms. Some minute vegetable and animal organisms are uni- 

 cellular also, but the larger forms are composed of many cells at- 

 tached to each other and interdependent on each other. This inter- 

 dependence is due to the fact that in such multicellular organisms 

 specialization has gone on, so that certain cells secrete digestive 

 juices, other cells are devoted to reproduction and so on. Amongst 

 the uni-cellular organisms, each cell performs all these functions 

 itself. 



Besides the cell-wall and its protoplasmic contents, both of 

 "which all bacteria possess, some bacteria possess flagella, or fine 

 hairlike whips only to be seen by special methods of staining, the 

 use of which is to propel the bacterium possessing them through 

 liquid — they are the swimming organs of the " motile " bacteria. 

 Bacteria without flagella are incapable of swimming and are called 

 non-motile. Motility, i. e., true purposive swimming movements, 

 must be distinguished from the Brownian movement. This is a 

 vibratory, sometimes circular motion which all minute particles sus- 

 pended in liquid are likely to show, due to the delicacy with which 

 they are held balanced in liquids, and the constant direct trans- 

 mission to the liquid of minute vibrations from outside. Particles 

 of resin, milk globules, dust particles, dead bacteria, etc., suspended 

 in liquid show this movement. In case of doubt between motility 

 and Brownian movement, the distinction can be based on the trans- 

 lation from place to place, resulting from motility, in contrast to 

 the absence of movement from place to place (except within the 



