THE MICROSCOPE IN BIOLOGY. 121 



way by one another, oftentimes stopping and hesitating, 

 yet always pursuing a determinate direction correspond- 

 ing to the long axis 'of the thread. They frequently be- 

 come stationary in the middle of their course, and then 

 turn round, but the greater number pass to the extreme 

 end of the thread, and then reverse the direction of their 

 movement." No physical or chemical action with which 

 we are acquainted will account for such motions, which 

 have no analogy in unorganized bodies. 



Changes of form are most strongly marked in the lower 

 forms of animal life, although occurring also in the sim- 

 pler vegetables, as the volvox. The Amoeba or Proteus is 

 typical of such changes, which have hence been termed 

 Amoeboid (Plate V, Fig. 90). When an Amoeba meets 

 another animal which is too slow to escape, it sends out 

 projections which encircle its prey; these coalesce, and 

 invest the whole mass with its bioplasm. It maintains 

 its grasp till it has abstracted all the portions which are 

 soluble, and then relaxes its hold. 



Amoeboid cells in higher animals rarely move so rapidly 

 as the Amoeba itself. Their motions are limited to a 

 gradual change of form or to the protrusion of processes 

 in the form of threads, or tuberosities, or tufts, which 

 either drag the rest of the body after them or are again 

 withdrawn. 



Cells of bioplasm rnay not only change their form, but 

 may wander from place to place by protruding a portion 

 of their mass, which drags the rest after it. The discov- 

 ery of wandering cells in the higher organisms, as man, 

 has opened quite a new and important field of physiologi- 

 cal and pathological research. 



The movements of bioplasm may be changed, acceler- 

 ated, retarded, or stopped by a variety of stimuli, mechani- 

 cal, electrical, chemical, and nervous. Gentle warmth and 

 moisture are necessary to their perfection.* 



* See Strieker's Manual of Histology. 



