THE CELL. 17 



CELL=DIVISION. 



We have learned that organs are composed of various structures, 

 called tissues. A tissue may be defined as "a group of similar cells 

 having similar functions." For example, muscular tissue is made up 

 of ribbonlike muscle-cells; mucous tissue of secreting, " goblet-shaped 

 cells; nervous tissue of ganglion-cells, with their numerous project- 

 ing dendrons, etc. 



By observation we notice a variety of tissues due to a variety of 

 kinds of cells; also that all tissues of a kind are not necessarily of 

 the same bulk, size, or weight. 



The chick contains, in its body, a number of organs of a definite 

 size and consistency. It has a head, limbs, muscles, a heart, lungs, 

 intestines, a liver, etc. We see, of course, that these organs are of a 

 size and weight in proportion to their age none of them large or 

 heavy. Upon examination, we find the tissues of the various organs 

 to be composed of cells such as we should expect them to contain; 

 that is, the muscles of muscle-cells, the bones of osseous cells, the 

 brain of ganglion-cells, etc. Furthermore, although cells are of dif- 

 ferent sizes and forms, yet there is very little difference in respect 

 to size between the cells of a particular tissue, as compared with one 

 another, or with those of the adult animal; for the size of every cell 

 is definite. 



When we observe the same animal one year after its birth, we 

 notice some striking differences: it is much larger and heavier, the 

 various organs are fuller, more compact, and show the effects of the 

 development as it approached maturity. The head, brain, muscles, 

 heart, lungs, intestines, etc., are all much larger and better developed 

 than those found in the small chick. However, if a microscopical 

 examination be made of the various tissues in this, the adult animal, 

 what do we find and how do the cells compare with those of the chick ? 

 Nothing remarkable in the individual cells themselves. The liver- 

 cells of the adult are no larger than those of the chick, nor are 

 the ganglion-, muscle-, or other cells. What we do perceive is a 

 great increase in the number of the cells in any particular tissue. 

 The liver and brain of the adult animal contain many more cells than 

 the same organs of the chick. Thus we see that there has been a 

 growth due, not to larger cells, but to a greater number of cells. 

 That is, the cells have multiplied. 



Similarly, as the infant passes through the various stages of boy- 

 hood, youth, and manhood, we say that he grows, for there is an 



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