THE VITAL PHENOMENA OF CELLS 



43 



FIG. 26. Mode 

 movement of an 

 Amoeba. The arrows 

 indicate the direction 

 and strength of the 

 protoplasmic c u r - 

 rents ; the crosses, the 

 resting places. For 

 the instant the prin- 

 cipal line of move- 

 ment is from H to- 

 ward V, but at any 

 other moment it may 

 turn toward L or R. 



which project quite independently of each other, or which 

 unite in the most complex sort of a network (Figs. 19, 20, 

 21, 26). 



Both phases of the movement of pseudopodia in 

 Amceba, namely expansion and contraction, are to be re- 

 garded as active processes, and the two are of equal im- 

 portance in ingesting food or in locomotion. In other 

 kinds of movement the most important phase consists in 

 a reduction of the surface of the cell i. e., in contraction. 

 To this category belong especially the movements of the 

 smooth and cross-striated muscles, to which we shall de- 

 vote proper attention in Chapter XV. 



3. The cell body of certain unicellular organisms pre- 

 sents specially differentiated contractile elements. Sten- 

 tor, for example, has in its outer sheath of protoplasm 

 smooth muscle fibrils running almost parallel; Vorticella 

 (Fig. 28) contains only a single smooth muscle fiber 

 composed of several fibrils. This leaves the body as a 

 thick cord, and is surrounded by an elastic membrane 

 to the inner side of which it adheres along a spiral course 

 from one end to the other. It serves as a stalk for the 

 attachment of the organism. If these contractile fibers 

 are roused to activity, they become shorter and thicker 



just like true muscles, and thereby change the form of the cell in a corre- 

 sponding way. 



4. In numerous unicellular organisms and in numerous cells of multi- ' 



cellular organisms, we find cilia 

 or flagella as special differen- 

 tiations of the cell body. These 

 are not temporary like pseu- 

 dopodia, but permanent struc- 

 tures of greater or less length 

 attached to the outer surface 

 of the cell. They are in con- 

 stant motion and occur on 

 every cell or on a great ma- 

 jority of the cells forming a 

 ciliated surface. If the cell 

 bears only a few (1 to 4) such 

 structures, they are called fla- 

 gella; if a larger number, they 

 are called cilia. In certain or- 

 ganisms they can be counted 

 by the hundreds and thou- 

 sands. 



FIG. 27. White blood corpuscles at 38 performing 

 amoeboid movements, after Max Schultze. The 

 different figures are all drawn from the same 

 corpuscle. 



Flagella are affixed either to 

 the anterior or the posterior end 



