.236 CLEAN MILK 



small area of the vibratory motion), seen in true Brownian move- 

 ment. Brownian movement of a particle combined with an actual 

 current in the liquid in which it is suspended may give an appear- 

 ance of actual translatory motion much like true living motility, 

 but can be distinguished from it in that the current movement is 

 all in some one direction, while individual bacteria showing motility 

 move in various directions. 



Certain bacteria at certain stages (generally when growth has. 

 occurred rapidly in one place for a considerable number of hours, 

 resulting in food exhaustion; or, more often, from the accumulation 

 of bacterial excreta in sufficient amount to obstruct further growth 

 ■by poisoning the living bacteria; or when drying, if it occurs 

 slowly) form spores, which are bodies developed in the cell (one 

 spore to one cell usually) containing the living protoplasm of the cell 

 in a dormant state, and having a spore wall of greater strength and 

 resistance than the wall of the parent cell. A cell containing a spore 

 is called a sporangium. The sporangium soon disintegrates, setting 

 the spore free. The spore remains alive but dormant until it reaches 

 a locality where conditions of food, temperature, etc., are favorable, 

 whereupon it germinates — i. e., the protoplasm within it becomes 

 active and begins to grow, escaping from the confines of the spore 

 wall and resuming existence in the identical form and activities of 

 its parent cell. Spores are particularly important to the milk bac- 

 teriologist, because the measures of sterilization, etc., wholly efficient 

 in the case of the ordinary bacterial cell, may fail if spores are 

 present, since the latter are much more resistant to heat, drying 

 chemical reagents, etc., than the adult bacterial cell. 



" Capsules " show themselves as oval rings or halos, surround- 

 ing bacterial cells. They are to be found more readily in some 

 •species than in others, although by no means constantly in any 

 ■species. It cannot be doubted that many appearances interpreted 

 as indicating capsules are really artefacts or optical illusions. 

 Nevertheless it is not improbable that a true bacterial substance 



