CHAPTER XXIV 

 CELLS AND TISSUES 



One of the most remarkable of the properties of "the 

 physical basis of life" is its tendency to build itself up into 

 a definite form and structure. Cattle, sheep, insect larvae, 

 and even parasite fungi and bacteria may all live upon 

 the tissue of the same kind of plant, each organism con- 

 verting the plant's tissue into its own peculiar kind of pro- 

 toplasm. We ourselves eat many kinds of meat and vege- 

 tables, but these foods are all converted into our own living 

 substance. The form assumed by each creature depends 

 very little upon the food it assimilates, but very much upon 

 the chemical and physical properties of the protoplasm 

 that is peculiar to its species. Each kind of protoplasm 

 tends to produce its own particular kind of organization, 

 be it man, dog, worm or plant. 



The body of a higher animal such as man is a wonderfully 

 complex mechanism, and in order to carry on its many 

 kinds of activities it is divided up into different parts or 

 organs each of which is especially fitted for its peculiar 

 work. Thus we have organs of locomotion, organs such 

 as the stomach, liver, etc., for the work of digestion, 

 organs of circulation for propelling and conducting the 

 blood, organs of respiration, excretion, and many others. 

 In a very simple animal such as the Amoeba these various 

 activities or functions are carried on by all parts of the 

 jelly-like body. There are no especial organs for the func- 

 tion of respiration or digestion or any other activities. 

 Food is taken in anywhere and digested anywhere in the 



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