MOTION 



39 



tions, this droplet of living substance sends out lobate processes into 

 the surrounding medium which are constantly increased in size and 

 length. These feelers, or pseudopodia, may be sent out in all direc- 

 tions, or may be restricted to one particular locality. In the latter 

 case, the entire mass of the cell may eventually be transferred into 

 one of these projections, occasioning in this way a slow onward streaming 

 of the protoplasm and its admixtures. This centrifugal movement, 

 however, may be changed at any moment into a centripetal one by 

 stimulation. The cell then assumes a nearly spherical outline, repre- 

 senting the state of contraction. 



This type of movement is not confined to the ameba, but is also 

 exhibited by the rhizopods, the egg cells of certain animals, pigment 

 and giant cells and the leukocytes of the blood. In the leukocytes it 



FIG. 9. AN AMEBA, SHOWING DIFFERENT STAGES OF MOVEMENT. (Verworn.) 



serves the primary purpose of engulfing nutritive particles, so that 

 these may be digested and assimilated by the living substance. It 

 is also made use of in ridding the body of toxic materials of all sorts, 

 this process having been designated by Metchnikoff as phago- 

 cytosis. In the plant cells in which this protoplasmic streaming is very 

 general, it serves the additional purpose of disseminating the food 

 substances. 



Ciliary Movement. 1 Cilia are cellular appendages possessing the 

 shape of slender, tapering hairs. Their length varies greatly in dif- 

 ferent animals. In the trachea of man, for example, they measure 

 0.003-0.005 mm. in length and 0.0003 mm. in thickness. Their num- 

 ber also varies. Some of the infusoria, such as paramecium, are beset 

 with several thousands of them, while an ordinary lining cell of the 

 digestive or respiratory passage may possess only several hundreds of 



1 Engelmann, in Hermann's Handb. der Physiol., 1879, i, 380; Putter, Ergebn. 

 der Physiol., i, 1903, and Verworn, Allg. Physiol., Jena, 1910. 



