66 THE CELL 



In addition there will be a special chapter on the process of 

 fertilisation. 



The cell may exhibit several kinds of movement, as is seen if an 

 extensive comparative study is made. We will here distinguish 

 between : (1) true protoplasmic movements ; (2) ciliary or 

 flagellar movements ; (3) the movements of the pulsating 

 vacuole ; (4) the passive movements and changes of shape 

 exhibited by cells. 



In addition to these four, there are a few special phenomena of 

 motion, of which it will be best to treat in later chapters, for 

 example, the formation of the receptive protuberance which appears 

 in the egg-cell in consequence of fertilisation ; the radiation figures 

 which are seen in the neighbourhood of the spermatozoon after it 

 has penetrated into the ovum, and those which occur during the 

 process of cell division, when the cell body splits up into two or 

 more parts. 



Protoplasmic Movements. Although it is probable that 

 movements take place in all protoplasm, yet in most cases, with 

 our present means of observation, they cannot be perceived on 

 account of their great slowness ; hence in only a few objects in 

 the plant and animal kingdoms can this phenomenon be studied 

 and demonstrated. The movement manifests itself partly in 

 changes in the external form of the cell, and partly in the arrange- 

 ment of the structure enclosed in the protoplasm, the nucleus, the 

 granules, and the vacuoles. 



These movements differ somewhat according as to whether they 

 are manifested in naked protoplasm, or in that which is enclosed 

 by a firm membrane. 



a. The Movements of naked Protoplasm. Small uni- 

 cellular organisms, white blood corpuscles, lymph corpuscles, 

 connective tissue cells, etc., exhibit movements which, in con- 

 sequence of their similarity to those seen in the Amoeba, are 

 termed amoeboid. 



If a lymph corpuscle of a Frog (Fig. 37) is observed under suit- 

 able circumstances, it is seen to undergo continual changes of 

 form. Small processes of protoplasm, the foot-like processes, or 

 pseudopodia, are protruded from its surface ; at first as a rule they 

 consist of hyaloplasm alone, but after a time granular protoplasm 

 streams into them. By this means the pseudopodia are increased 

 in size ; they become broader, and may in their turn extend new, 

 more minute processes from their surface. Or the protoplasm may 



