60 



STRUCTTJEAL UNITS OP HUMAN TISSUES 



each set of cells performs special work just as in a cotton 

 mill, spinners, weavers, and others have their special tasks. 

 Reproduction of cells. — Before we leave the tissues, one 

 feature of cell action which they all share must be con- 

 sidered more fully. Like the parts of a machine, the parts 

 of our body are continually being broken down by use. 

 To remedy this there must be continual repair or building 

 up. Otherwise the body, like the machine, soon becomes 



Fig. 20 — Beproduction of a cell; A, diagram of a cell, showing centrosome (o), 

 and nucleus (n) consisting of nuclear membrane inclosing granular substances 

 in which are a spherical nucleolus and irregular masses of chromatin ; £ to ^, 

 changes in the course of reproduction, 



worn out and useless. How is this brought about? The 

 answer to this question is found in the discovery of a very 

 peculiar property of protoplasm called reproduction. This 

 means that an old cell has the power to produce new cells 

 out of its protoplasm. The manner in which it does this 

 is illustrated in Figure 20 ; thus a single cell or egg is able 

 by repeated divisions to produce all the tissues of a body, 

 and a tissue is able to renew its structure to maintain its 

 efficiency. When the protoplasm of an individual loses this 

 power the individual soon becomes worn out and dies. 

 Life depends upon the reproductive power of protoplasm. 



