"JO HISTOLOGY. 



independent movement, we notice that they constantly change 

 their form. This is seen most easily if we make outline sketches 

 of the cell at short intervals and compare them (Fig. 3). This 

 motility serves not only to change the location of the cell, but 

 also to aid in the acquiring of nourishment. The pseud* (podia 

 surround the very fine foreign bodies with which they come in 

 contact, draw them into the cell, and if they are digestible use 

 i hem for the nourishment of the organism. Upon motility of 

 this sort, a great proportion of the unicellular animals (e. g., 

 amoeba) and many kinds of cells of higher animals (e. //., white 

 blood-corpuscles) are dependent. 



(b) The second kind of motility, the so-called ciliary move- 

 in << nt, is brought about by shorter or longer processes of the cell 

 substance, the so-called cilia or flagella. It seems that this 

 movement also is independent of the nucleus. Cilia are of a 

 more permanent nature than pseudopodia. The latter may be 

 pushed out or withdrawn, while the former are developed by a 

 special differentiation of one part of the protoplasm and remain 

 as they are, without changing as the cell moves. 



(c) Muscular contraction is movement which is due to a 

 special differentiation of the protoplasmic network, and serves 

 to move not so much the cell itself, as the organism to which 

 the cell belongs. 



(d) We distinguish a twofold movement which is present in 

 the protoplasm in the cell body : circulation and rotation. These 

 movements are made visible to the eye in consequence of the fine 

 granules present in the protoplasm, and are observed especially 

 in plant cells, seldom in animal cells. If the grannies move in 

 one direction around under the cell membrane, we have to do 

 with rotation. If, however, the movement is from the periphery 

 to the centre in one part of the cell, and in the opposite direction 

 in other parts, we have circulation. This occurs in all plant 

 cells where the protoplasm contains vacuoles filled with fluid. 

 In these cases strands of protoplasm connect the central peri- 

 nuclear protoplasm with that at the periphery; and in these 

 strands we often see two streams of granules flowing in opposite 

 directions. 



