MOVEMENTS OF CELLS. 11 



the slide like a gelatinous body ; or, lastly, it may contract up 

 into a resistant ball. 



Protoplasm may therefore be said to be fluid, or solid, or 

 gelatinous. Its states of aggregation are subject to constant 

 change, and none of the ordinary terms employed for this pur- 

 pose will be in all cases accurate. 



Protoplasm is termed a living substance, and the application 

 of this term is based upon its exhibiting the sum of those 

 phenomena which we have learned by experience to be cha- 

 racteristic of living animals. These phenomena are active or 

 spontaneous movement, nutrition and growth, and the capa- 

 bility of reproducing its like. 



The movement of cells is easily to be seen. The changes 

 of which it is the result take place in so short a space of time 

 that they may be followed with the eye. The growth of the 

 cells is a process of a slower nature, and cannot be directly 

 observed ; nor has any one, as yet, been able to place the cells 

 under the microscope, under such favourable conditions as to 

 witness their increase in size. We must therefore be led by 

 analogy to the conclusion that this really takes place. 



Various observations have been directly made on the nutri- 

 tion of unicellular animals. It may be seen how they take up 

 foreign bodies, and nutritious material, into their bodies, and 

 some of the changes of the material introduced may be followed. 

 It is difficult to observe the mode of nutrition that occurs in the 

 cells of the compound animal body, because the nutritive 

 materials are brought to them in the form of solution in the 

 juices of the animal body. Moreover, the processes by which 

 the dissolved substances penetrate the cells is concealed from 

 our observation. 



The act of reproduction depends on two separate processes ; 

 first, on the growth of the mother-cell, and secondly, on the 

 detachment of the daughter-cell (birth). The latter alone is 

 subject to direct observation, and usually this only is under- 

 stood when reproduction is under consideration. 



PHENOMENA OF MOVEMENT IN CELLS. We conclude that 

 movement occurs in cells, either from certain movements of the 



