122 GENERAL PRINCIPLES OF ZOOLOGY. 



occur in the vegetable kingdom, still they reach no high 

 development ; indeed we may say they become more and 

 more inconspicuous the higher the plants develop ; in the 

 animal kingdom, on the contrary, they are unfolded in 

 extraordinary completeness, and lie at the basis of its most 

 characteristic features. 



Vegetal Organs. 



A. Organs of Assimilation. 



Assimilation Denned. If the term assimilation is 

 used in its widest sense, one must speak in this connection 

 of all the contrivances which are found in the animal body 

 for rendering growth possible during the period of pro- 

 gressive development, and also during mature life for com- 

 pensating for the loss of energy connected with each period 

 of functional activity, in order to preserve to the body its 

 functional capability. In each period of functional activ- 

 ity organic compounds are oxidized, or, to speak figura- 

 tively, burnt. Compounds which are especially rich in 

 carbon and relatively poor in oxygen, containing especially 

 hydrogen (usually also nitrogen and sulphur), in the 

 presence of oxygen are changed by oxidation into car- 

 bonic acid, water, and various nitrogenous products, like 

 urea, uric acid, etc. A compensation takes place, for 

 not only is the useless substance removed, but also com- 

 pounds of oxygen and materials rich in carbon are fur- 

 nished to the tissues to replace the material oxidized. 



Assimilation in Animals. In lowly-organized animals 

 all the processes connected with compensative assimilative 

 changes take place through the agency of one and the 

 same organ, the digestive tract ; but in the higher animals, 

 through specialization, normal assimilation has come to ex- 

 hibit a definite series of separate phenomena. Between 

 the lower and the higher animals there are easily com- 

 prehended, intermediate conditions where specialization 

 has halted at an earlier or a later stage. 



Different Organs of Assimilation. Every assimila- 



