THE FUNDAMENTAL PHYSIOLOGICAL ACTIONS 23 



the halves of 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 another, giving rise to 

 new beings like themselves, known as reproduction, are possessed 

 by all kinds of living beings, whether animals or plants. There is, 

 however, this important difference 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 breaking down and being removed as long as life 

 lasts, and the loss must be made good if any of them is to con- 

 tinue its existence. The 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 com- 

 posing 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 invariable in 

 living beings as the building up of new complex molecules referred 

 to above. It is associated with the assumption of uncombined 

 oxygen from the exterior, which is then combined directly or in- 

 directly with other elements in the cell, as, for example, carbon, 

 giving rise to carbon dioxid, or hydrogen, 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 producing movements. The carbon dioxid 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 nutri- 



