20 



ADVANCED PHYSIOLOGY 



f.D. 



Repair and growth are brought about by one and the same 

 process, the difference being that in repair, new material is 

 added only as fast as the old wears away, while in growth 

 new material is formed in excess of that which is worn out,, 

 the excess constituting the growth. 



Cell Division. Most living cells have the power of self 

 division as shown in Fig. 16. This results in two cells like the 

 original. In the human body, however, the cells do not 

 separate from each other, so that by continued repetition 

 of this process a large mass of cells 

 is produced. 



Reproduction. Every animal begins 

 its life as a single cell an egg. This 

 cell repeatedly divides, the many cells 

 remaining attached to each other. 

 Thus the growth of an animal to 

 adult size is not due to the growth in 

 size of the individual cells making up 

 its body but to increase in their number. 

 The cells of the adult are not materially 

 larger than those of the young. In 

 time one or more of the cells from the 

 body of the adult may be set apart as 

 eggs, the process outlined above being 

 started over again. This derivation 

 of new individuals from single cells of 

 a preceding one is called reproduction. 



The Cell as a Unit. We have thus 

 seen that the body is made of organs, 



that the organs are made of tissues and that the tissues are made 

 of cells. Is it possible to carry this division further? To this 

 question we must reply that, so far as yet known, the cell is 

 the final unit. It is true that the cell has parts cell wall, nucleus, 

 cell substances etc. but no one of them can live by itself, 

 while a complete cell may be an independent body and live an 



FlG 14. A CELL SHOWING 



ITS INTERNAL STRUC- 

 TURE 



C.TF., cell wall; C.L., cell 

 liquid; C. R., cell reticulum; 

 Nuc., nucleus; Nucl., nucleo- 

 lus; Ch., chromatin; Cn. t 

 centre-some; Vac. vacuoles; 

 pl., plastida (pigment, chloro- 

 phyll, etc.); F. D., fat drops 

 (starch, gum etc. may be 

 similarly present in plant 

 cell). 



