CHANGES OF FORM IN CELLS. 15 



Their movements are either limited to gradual change of 

 form, or to the protrusion of processes which either drag the 

 rest of the body after them, or are again withdrawn. The 

 processes may assume the form of threads, swellings, tuberous 

 elevations, or broad flattened projections or tufts, and may pre- 

 sent the greatest diversities of form. 



If the alterations in shape are desired to be accurately noted, 

 the cell must be placed upon a slide, or on a piece of tissue, or 

 may even be attached to the cover ; for if the cells swim in 

 fluid, it is possible they may turn, and thus present different 

 surfaces to the observer. No conclusion can be drawn respect- 

 ing the life of a cell, from the observation of a single change 

 of form, since it is impossible to ascertain whether some 

 unknown physical influence may not have wrought the change. 

 Those alterations of form only which may be perceived in the 

 object when the field of view is stationary, and when the object 

 is adherent to the slide, and which are frequently repeated, 

 enable us to determine the presence of life in it. 



Conversely also, we must not consider a quiescent proto- 

 plasmic mass as necessarily dead, even if we are unable artifi- 

 cially to excite movement by means of reagents. The proto- 

 plasmic substance may possibly be encapsuled when it is not in 

 a state to change its form, and even if it be naked, some 

 unknown cause may hinder its movements. Hence, it cannot 

 be said that the salivary corpuscles are dead, because as a rule 

 they do not change their external form. 



Protoplasmic corpuscles are not only able to change their 

 form, but their place also ; they can wander. This is accom- 

 plished by the protrusion of one portion of their mass, which 

 drags the rest after it. If such alterations of form are re- 

 peated several times, and in the same direction, locomotion is 

 effected. 



It must not be overlooked that entire cells may exhibit 

 vibratory movements in fluids obviously subject to the laws 

 of the Brunonian molecular movements. The stellate blood 

 corpuscles of mammals, for example, do so as a rule. Such 

 vibratory movements are to be clearly distinguished from the 

 migrations of cells. Cells can only move from place to place 

 when resting on a firm basis. They may swim in fluids, owing 



