22 THE HUMAN BODY. 



the original centrosome, probably pass out of the new nuclei 

 and become the attraction particles of the new cells. 



The phenomena of karyokinesis show clearly that in spite of 

 its small size the animal cell is a complicated structure, made 

 up of very distinct parts possessing very distinct properties 

 and no doubt very different functions. 



Assimilation: Reproduction. The two powers, that of 

 working up into their own substance materials derived from 

 outside, known as assimilation, and that of, in one way or an- 

 other, giving rise to new beings like themselves, known as re- 

 production, are possessed by all kinds of living beings, whether 

 animals or plants. There is, however, this important differ- 

 ence between the two: the power of assimilation is necessary 

 for the maintenance of each individual cell, plant or animal,. 

 since the already existing living material is constantly break- 

 ing down and being removed as long as life lasts, and the loss 

 must be made good if any of them is to continue its existence. 

 , *Fhe power of reproduction, on the other hand, is necessary 

 only for the continuance of the kind or race, and need be, and 

 often is, possessed only by some of the individuals composing it. 

 Working bees, for example, cannot reproduce their kind, that 

 duty being left to the queen-bee and the drones of each hive. 



The breaking down of already existing chemical compounds 

 into simpler ones, sometimes called dissimilation, is as inva- 

 riable in living beings as the building up of new complex mole- 

 cules referred to above. It is associated with the assumption 

 of uncombined oxygen from the exterior, which is then com- 

 bined directly or indirectly with other elements in the cell, as, 

 for example, carbon, giving rise to carbon dioxide, or hydro- 

 gen, producing water. In this way the molecule in which the 

 carbon and hydrogen previously existed is broken down and 

 at the same time energy is liberated, which in all cases seems 

 to take in part the form of heat just as when coal is burnt in 

 a fire, but may be used in part for other purposes, such as pro- 

 ducing movements. The carbon dioxide is usually got rid 

 of by the same mechanism as that which serves to take up the 

 oxygen, and these two processes constitute the function of 

 respiration which occurs in all living things. Assimilation 

 and dissimilation, going -on side by side and being to a certain 

 extent correlative, are often spoken of together as the process 

 of nutrition : the assimilative or chemically constructive pro- 

 cesses are also named anabolic, and the dissimilative katabolic. 



