CELLS AND THE CELL THEORY 37 



and bone contain in a marked degree lifeless cell walls from 

 which the living matter is either wholly withdrawn, as in the 

 hair, or remains only in a relatively small amount, as in 

 bone and cartilage. 



Other Substances in a Cell. Cells may contain other bodies 

 than those already described, which cannot be regarded, how- 

 ever, as essential to cell life, since they are not characteristic 

 of all cellular structure. Some of these are called plastids (Fig. 

 9 p) , and seem to grow and divide and to be handed on from 

 one cell generation to the next. Examples of such plastids are 

 the chlorophyll bodies in plant cells, or vacuoles in some animals. 

 Other bodies included in cells are purely passive bodies which 

 seem to be functionless, inert, excreted substances, not growing 

 and not handed down from generation to generation. 



CELL FUNCTIONS 



The cell with its protoplasm and nucleus contains all of the 

 parts that are necessary for life, and, so far as we know, nothing 

 simpler than a cell is capable of carrying on all the functions of 

 life. If this be true, we are justified in saying that the ideal cell 

 we have been describing is the simplest bit of structural machinery 

 that can manifest all the functions of life. All living organisms, 

 animals and plants alike, are either single cells (unicellular) or 

 complexes of cells (multicellular), and the life of the organism 

 as a whole is thus the combined life of its individual cells. 



Definition of a Cell. To sum up, then, we may say: A cell is 

 a combination of a bit of protoplasm (cytoplasm) with a nucleus, 

 and it is the simplest structure known to show the phenomena 

 of life. 



HISTORY OF THE CELL DOCTRINE 



The development of the cell doctrine may, for convenience, be 

 divided into three periods : 



1. The early conception of the cell, 1839 to 1861. 



