12 



THE ANIMAL CELL 



[GIL II. 



A streaming movement is not infrequently seen in certain of 

 the protozoa, in which the mass of protoplasm extends long and 

 fine processes, themselves very little movable, but upon the surface 



FIG 11. Human colourless blood-corpuscle, showing its successive changes of outline within ten 

 minutes when kept moist on a warm stage. (Schofield.) 



of which freely-moving or streaming granules are seen. A gliding 

 movement has also been noticed in certain animal cells ; the motile 

 part of the cell is composed of protoplasm bounding a central mass ; 

 by means of the free movement of this layer, the cell may be 

 observed to move along. 



FIG. 12. (A) Young vegetable cells, showing cell-cavity entirely filled with granular protoplasm 



enclosing a large oval nucleus, with one or more nucleoli. 



(B) Older cells from same plant, showing distinct cellulose-wall and vacuolation of proto- 

 plasm. 



In vegetable cells the protoplasmic movement can be well seen 

 in the hairs of the stinging-nettle and Tradescantia and the cells of 



Vallisneria and Chara ; it is marked 

 by the movement of the granules 

 nearly always imbedded in it. For 

 example, if part of a hair of Trade- 

 scantia (fig. 13) is viewed under a 

 high magnifying power, streams of 

 protoplasm containing crowds of 

 granules hurrying along, like the 

 foot-passengers in a busy street, 

 are seen flowing steadily in definite 

 directions, some coursing round the 

 film which lines the interior of 

 the cell-wall, and others flowing 

 towards or away from the irregular 

 mass in the centre of the cell-cavity. Many of these streams of 

 protoplasm run together into larger ones and are lost in the 



FIG. 13. Cell of Tradescantia drawn at suc- 

 cessive intervals of two minutes. The cell- 

 contents consist of a central mass connected 

 by many irregular processes to a peripheral 

 Him, the whole forming a yacuolated mass 

 of protoplasm, which is continually changing 

 its shape. (Schofield.) 



