40 PROTOPLASM AND THE CELL 



surface of layers of ciliated epithelium (as in the bronchi and Fallopian 

 tubes), or as flagella attached to the spermatazoa, or about such uni- 

 cellular animals as paramecium. In the former case the function of the 

 cilia is to move along light substances which adhere to them (for example, 

 dust particles in case of the bronchi, ova in the Fallopian tubes). Fla- 

 gella move the cell of which they are a part through its liquid environ- 

 ment, or else, as in hydra, move masses of liquid within the animal. 

 The cilia are a somewhat differentiated part of the protoplasm proper, 

 and in cases where a cell-wall exists they extend through pores in the 

 latter. The motion of cilia consists alternately in a forcible contractile 

 erection and a slower, more or less passive relaxing movement. The 

 position of the cilium in the former case is perpendicular to the general 

 surface of the cell, and in the latter more or less parallel to it. It is largely 

 through the difference in the velocity of these two phases of motion that 

 their motor function is possible. The movement of a surface of the cilia 

 shows a perfect rhythm, best described by the partly schematic figure 

 (Verworn). In many cases the movements of the cilia, and of the flagella 

 especially, are complicated combinations of spiral, funnel-shaped, whip- 

 movements and of others too complex to be indicated. The principle 

 of action is in all cases similar namely, according to Verworn, that "a 

 contractile side (of a cilium) contracts from the cell-body outward, and 

 thereby the opposite side is extended; in the phase of expansion (or 

 relaxation) the latter by its elasticity brings the cilium back into the 

 position of rest. According to the relative positions of the contractile and 

 the passively extended substances, there results a movement in a plane 

 or a more complicated form." The mechanism by which the rhythm 

 of movement on a surface of ciliated epithelium is kept up with such 

 perfect regularity and adaptation to the needs is not understood. The 

 existence of a nervous coordinating apparatus has not been proved, so 

 that it depends on impulses passing, in manner and route unknown, 

 through the protoplasm itself, and from cell to cell indefinitely. Such 

 subjects as this, the typical movements of the basal protoplasms, which 

 are almost impossible of being understood by mere description, at once 

 illustrate and prove the importance of actual laboratory work in elemen- 

 tary biology as a part of every course in physiology. 



The third variety of active contraction-relaxation movement exhibited 

 by protoplasm that may be mentioned is muscular movement. The 

 needs of the evolving animal world rapidly became complex at an early 

 period, and demanded evidently a closer, stronger, and more adaptable 

 means of motion than was possible from any sort of bioplasm then 

 existing. Thus (so runs the teleological theory of evolution) muscle was 

 made to develop, with its complex and extraordinary functions. In 

 general terms this function is always either to draw closer together two 

 parts of an organism or else to diminish the caliber of a tube or of a hollow 

 viscus. Among the lower and simpler animals various transition-stages 

 between epithelium and muscle cells may be found. Sometimes, for 

 example, one sees the upper part of an epithelial cell developed into a 



