Life as a Physical Phenomenon. 157 



upon the principal tiss lie of man himself. Thus far we have 

 spoken only of cells that retain very nearly the typical, 

 spherical forms ; but many cells are so modified that a casual 

 examination detects no relation to the type. The cell is 

 modified in form according to the purpose it is to serve in 

 the economy of the individual. In the fibre of the linden 

 or oak it is long and flexible, more like a thread than what 

 we have figured as a cell. In what are called fibrous tis- 

 sues too in the bodies of animals it is often drawn out in 

 the form of a long cylinder. Perhaps we may be able to 

 trace the transition from the round cell to the cylindrical 

 one so that we may keep the relationship in view. 



Let us make a hollow sphere of india-rubber in imi- 

 tation of a round cell, the rubber taking the place of 

 cellulose. Let the sphere be large enough to illustrate the 

 changes we desire to witness and the walls thin enough to 

 be transparent. In the centre is a fluid, and within it 

 some elastic substance which may represent the dot. Now 

 let us fasten two fine hooks, one to one side and one to 

 the other of our elastic, transparent sphere. Draw upon 

 the hooks and do we not see that the rounded body be- 

 comes pointed ? Draw a little more, it is fusiform ; still 

 more, it is like a fibre, and now we can only see it as a 

 tube. The first, or rounded form is found in blood cor- 

 puscles in our own vessels, and discs of essentially the 

 same structure are also found in the mucous membrane lin- 

 ing the mouth, in the skin and in other tissues of the body, 

 and we have already seen how slight is its modification in 

 muscular fibre. The other forms are found in fibrous tis- 

 sues. The long tubular form is common in woody fibres. 

 The fibre of cotton is an example of a cell greatly extended. 

 It might be shown, had we the time and were it neces- 

 sary, that every part of a plant or an animal is built of 

 cells, but the proposition that such is the case will doubt- 

 less be accepted by all. 



