62 The Story of the Bacteria 



these cell communities are all co-ordinated so 

 as to act in harmony. We have seen that 

 these cell communities which make up this 

 wonderful mechanism are all originally de- 

 rived from a single living cell, the ovum. 



What this mysterious thing is which we call 

 life, which from the original cell, the ovum, 

 is imparted to all the myriad specialized cells 

 which springs from it as the body grows; what 

 it is which determines that from one of two 

 cells which under the most powerful of micro- 

 scopes look exactly alike there shall develop 

 a man, and from the other an animal, we 

 simply do not know. We theorize, we specu- 

 late, we draw analogies, we give names, but 

 at the end we conclude that we must wait still 

 for more light. We do know, however, that 

 this self -built cellular mechanism, the body, 

 which is alive, has in it the power of self- 

 renewal: the power, when once started, to 

 go on doing the various things for which it is 

 fitted for a time, provided the proper external 

 and internal conditions are maintained. But 

 sooner or later the machinery begins to creak 

 and tremble, sometimes in one part, sometimes 



