to activities of its constituent units, the living cells. 

 Nothing will admit of so little manipulation, and all of the 

 lore of physical science must be drawn upon for the care- 

 ful disentanglement of tht maze of phenomena exhibited by 

 a single living cell. A tiny speck of jelly, more than three- 

 fourths water, it may move, feed, digest, grow, reproduce, 

 and respond to all manner of changes in its surroundings, 

 but the slightest change in a single condition of its en- 

 vironment may snuff out its life. 



But since we must begin somewhere, and since it is the 

 phenomena of the living cell which we are to study, let us 

 start by dividing these phenomena roughly into two classes: 



1. Those phenomena which are characteristic of living cells 

 in general, and which are exhibited by most or all living 

 cells. Such are respiration, growth, irritability. 



2. Those phenomena which are peculiar to certain types of 

 living cells, and not exhibited by the great majority of 

 cells. Such are ameboid and ciliary movement, photosyn- 

 thesis, secretion, and perhaps contractility. 



Now while both of these classes of phenomena must be stud- 

 led and are of great importance on account of their wide- 

 spread occurrence and significance, it seems that in the 

 present state of our knowlege of living cells, ignorant 



(5) 



