88 ZOOLOGY 



ficial cells of the body, or consist of a mixture of cells and in- 

 ter-cellular substance. Their nature and arrangement are 

 profoundly important in determining the distribution of the 

 other more active organs. 



6. Growth and reproduction are the outcome of the nutri- 

 tive processes. Growth is increase in mass; reproduction is the 

 production of new individuals from old. Reproduction always 

 involves cell division and may be asexual or sexual. The latter 

 normally involves two parents. In it two cells, which may be 

 either similar or dissimilar, must unite before development will 

 proceed. 



7. The nervous and muscular systems are closely related 

 in function. Their united work is to receive, coordinate and 

 respond to the external or internal stimuli affecting the animal. 

 The structures to receive stimuli (end organs) are largely super- 

 ficial; the coordinating and controlling parts (central organ) are 

 deep-seated, thereby securing protection and a central position; 

 the muscular system must have definite relations with the hard 

 parts upon which it acts. Thus arises the necessity of connec- 

 tives or nerves between the various portions. 



8. The sense organs represent areas of the epithelium which 

 are peculiarly adapted to the reception of some one of the forms 

 of stimulus to which animals are subject, supplemented by a 

 more or less complex apparatus which serves to intensify or 

 modify the original stimulus. The sense organ determines the 

 kind of stimulus which may be received, but the central nervous 

 organ determines the nature of the sensation which results, and 

 the response. 



9. If cells and organs become differentiated, and specialized 

 in the division of labor, it becomes doubly essential that they be 

 brought together again in a complete unity if there is to be an 

 efficient individual. This reintegration of diverse parts is (i) 

 physical, by means of connective and supportive tissues; (2) 

 chemical and nutritive, by means of blood and lymph; and 

 (3) cooperative, by means of the nervous system. 



10. Organs with different origin which by reason of similar 

 function have come to look alike are said to be analogous. 

 Organs with similar origin and structure, even though they may 



