38 



PROTOPLASM AND THE CELL 



it is alive. So strong is the instinctive feeling in all animals that squirrels 

 will sometimes run over a motionless figure as over a fallen log. Motion, 

 though unsatisfactory to define either by common sense or by meta- 

 physics, is a quality of protoplasm inherent in the matter of which it is 

 made. Probably movement inheres as characteristic in the structure 

 of the so-called biomolecule. The most important kinds of movement 

 are produced by the active contraction of the protoplasm. These are 

 three in number namely, streaming, ciliary, and muscular. The first 

 occurs in uhdifferentiated bioplasm and in the leukocytes, the second in 



FIG. 11 



1 



FIG. 12 



A cell of ciliated epithelium from a mollusk, 

 (Engelmann.) 



Paramecium caudatum, Ehr. X 250. (Conn.) 



cells which have evolved into a 

 special sort of epithelium, and the 

 last in highly specialized cells 

 adapted to this one function of 

 causing motion. All of these are 

 obviously dependent on the fluidity 

 of the living matter. 



The streaming movements of protoplasm are of much interest, and in 

 a sense are the type of which the other two are only variations. These 

 movements may be seen in the ameba (Fig. 1), apparently only a speck 

 of gelatinous liquid, comparatively at rest at first, somewhere on the 

 slide of the microscope. Soon a slight bulging occurs at the edge of 

 the cytoplasm, and this gradually increases and becomes a projection, 

 extending outward as much as the former diameter of the cell, or even 

 much farther. If the animal be bent on progression (for example, to 

 escape from too bright a light), the whole body of protoplasm slowly 



