THE METAZOA 



waging a ceaseless deadly war against 

 them ever since. We have already seen 

 that so soon as a tree or large animal dies, 

 its lifeless body is at once attacked by 

 unicellular forms in a most effective 

 fashion, as if they would get rid of every 

 trace of such multicellular things. But 

 their activities are not only post-mortem. 

 Instead the unicellular forms remain ever 

 on the watch to break through every bar- 

 rier raised by the metazoa for self-defence. 

 The chief barrier is the thin basement 

 membrane, as it is called, on which grow 

 the cells of the external skin and of the 

 internal skin or mucous membrane. So 

 long as these covering layers of cells are 

 healthy, billions of the enemy may accu- 

 mulate there without effecting anything. 

 The instructed surgeon, however, well 

 knows that he cannot make the smallest 

 incision or sometimes even puncture of 

 this protective envelope without peril to 

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