MECHANICAL MOVEMENTS IN CELLS. 73 



but that all the particles, as in a liquid, are capable of mutual rearrange- 

 ment of position. Further, the stimulus to motion is not invariably 

 applied from without, but may be self-originating in the interior of the 

 mass. Protoplasm is thus contractile, irritable, and automatic. 



Protoplasm, wherever found, is a transparent, colorless, apparently 

 homogeneous mass, refracting light somewhat more strongly than water, 

 but less than oil. Where protoplasm may be separated into layers, as in 

 the ectosarc and endosarc of some of the lower animalcules, protoplasm 

 may be doubly refractive, and when the direction of motion of the 

 protoplasm is constant the optic axis coincides with the line of motion. 

 Protoplasm, as previously indicated, possesses considerable power of 

 imbibition, moderate cohesion, and great extensibility, the degree of each 

 of these physical attributes varying in different forms of protoplasm, 

 and at different times and under different conditions for the same 

 protoplasm. 



Protoplasm also usually contains a variable number of granules of 

 foreign matter, which are passive in the motions of protoplasm, but 

 which themselves may manifest oscillatory movement (Brownian motion). 

 The reaction of protoplasm is usually faintly alkaline or neutral. 



Protoplasm may produce movement by means of prolongations of 

 cells, or by the contraction of organized -matter resulting from the 

 metamorphosis of cell-contents. We have therefore to consider — 



First. — Protoplasmic and cellular motion, whether limited by a cell- 

 membrane, or occurring in free protoplasm. 



Second. — Motion of the protoplasmic prolongations of cells, as seen 

 in ciliary movement ; and 



Third. — The contraction of substances resulting from the metamor- 

 phosis of cell-contents, as seen in muscular tissue. 



1. Movements in Protoplasmic Contents of Cells.— In addition to 

 the Brownian movement, or oscillatory movement of granules which is 

 seen whenever minute particles, whether organic or inorganic, are sus- 

 pended in a fluid, and which are simply due to varying currents produced 

 by differences of temperature, the motion in the protoplasmic contents 

 of cells may be either circulatory (cyclosis) or ma}' result in changes of 

 form. Circulatory movements are seen in numerous vegetable cells, , 

 particularly when the protoplasmic contents have decreased somewhat 

 in amount so as not to fill the entire interior of the cell ; the protoplasm 

 is then heaped up against the walls of the cells, and sends prolongations 

 across the interior. These cell-contents may then manifest movements, 

 either of changes of form or of circulation of starch granules, etc., 

 which are imbedded in the protoplasm. 



If a cell of the Tradescantia virginica is examined under the micro- 

 scope, the protoplasmic cell-contents will be found to be arranged in the 



