Chap. I.] 



THE CELL. 



ity of the cell is promoted by warmth, electrical or other 

 stimulation, the action of certain drugs, etc. 



(3) The most obvious physical changes that can sometimes 

 be seen in living protoplasm, by the aid of the microscope, 

 are those which are termed "amoeboid." This term is derived 

 from the amoeba, a single- 

 celled organism which has 

 long been observed to exhibit 

 spontaneous changes of form, 

 accompanied by a flowing of 

 its soft semi-fluid substance. 

 By virtue of this property, 

 the cells can move from one 

 place to another. If one of 

 these cells be observed under 

 a high power of the micro- 

 scope, it will be seen gradu- 

 ally to protrude a portion of 

 its protoplasm ; this protru- 

 sion extends itself, and the 

 main part or body of the cell 

 passes by degrees into the 

 elougated protrusion. By a 

 repetition of this process, the 

 cell may glide slowly away 

 from its original situation and 

 move bodily along the field 

 of the microscope, so that an 

 actual locomotion takes place. 

 When the surface of these 

 free cells comes in contact Fig. 3. — .4 to i7, Consecutive Stages 

 with any foreign particles, the «*' Cell-Division, with indirect Divi- 



•^ . . SIGN OF THE NUCLEUS. (Diagrammatic.) 



protoplasm, by virtue of its 



amoeboid movements, tends to flow round and enwrap the 

 particles, and particles thus enwrapped or incepted may then 

 be conveyed by the cell from one place to another. 



The nucleus. — The nucleus of a cell is directly concerned in 

 the nutrition and in the reproduction or division of the cells. 

 In dividing, the nucleus passes through a series of remarkable 

 changes, which are too complicated to be studied here. (See 



