140 SYSTEMATIC ZOOLOGY. 



Protoplasm has the power of spontaneous motion. 

 Under favorable conditions we can see its particles chang- 

 ing their relative position, or we may see the mass move as 

 a whole. It moves also in response to external influences, 

 or, as the physiologist expresses it, it reacts to stimuli. 

 Thus some protoplasm will turn to light, other kinds will 

 try to avoid it. Heat, up to a certain degree, will increase 

 its action, while electricity will cause it to contract. 



Protoplasm has the power of reproduction, by which we 

 mean that portions can separate from the parent mass and 

 can then carry on all the processes which could be per- 

 formed before the separation took place. 



These and a number of other features not so easily de- 

 scribed are characteristic of protoplasm, and they occur in 

 no non-living substance. They are, too, the phenomena 

 of life, and hence protoplasm has been aptly termed the 

 physical basis of life. 



THE CELL. 



In most animals and plants the protoplasm is not a 

 continuous mass, but is divided into small particles called 

 cells, which, since they are the elements of which the 

 organism is composed, are the morphological units of 

 structure. It is almost self-evident then that in order 

 clearly to understand either the structure or the various 

 phenomena of either animal or plant we must have a 

 knowledge of the cell. Hence it is that within recent 

 years the study of the cell cytology has attained great 

 prominence. 



A cell may be defined as an individual particle of proto- 

 plasm with a specialized portion, the nucleus, in its in- 

 terior (fig. 1). Nucleus and protoplasm differ in many re- 



