PROTOPLASM. 9 



non-living substances ; but, so far as we know, it is never pro- 

 duced except as the result of the growth and division of 

 antecedent protoplasm. The protoplasmic or living material 

 in an organism is normally composed of a number of unit- 

 masses called cells (see Chapter III). These unit-masses of 

 protoplasm are in some degree independent of one another, 

 because normally each tends to form a wall about itself; and 

 yet it is highly probable that the whole protoplasm of an ani- 

 mal is physically continuous by means of delicate connections 

 between the units. The life of the cells is not quite the same 

 thing as the life of the organism to which they belong, for 

 in animals composed of more than one cell a cell may die with- 

 out involving the death of the animal. The protoplasm of the 

 cell may also retain life for a time after separation from the 

 living animal or after the animal as a whole has ceased to live. 

 This may be seen in the fact that the colorless corpuscles of 

 the blood, which are regarded as cells of the body, may con- 

 tinue to move and show other evidences of life after removal 

 from the body. 



15. Protoplasm. The protoplasm is the strictly living, 

 active material of organisms. The term is sometimes applied 

 so as to embrace non-living substances which are found . in 

 close connection with that which is thought to be " alive." 

 Even limiting the term protoplasm to the living matter, we 

 must avoid regarding it as a single substance of definite com- 

 position. We should rather consider it a very complex mixture 

 of substances, each of which is a highly complex compound. 



While protoplasm seems fundamentally the same in plants 

 and in animals, there are yet important differences; and it is 

 probable that the living matter of different animals, and even 

 of different parts of the same animal, is diverse in chemical or 

 physical structure. This fact, rather than any possible dif- 

 ference in the " life-force " itself, seems responsible for the 

 diversity of powers of different organisms and of different 

 organs. 



