THE CAUSE OF INFECTIOUS DISEASE. 



hepaticum three parasitic and two free stages 

 are known. 



We know that among the rusts or Uredinecz 

 there occur upon grain both summer spores or 

 uredospores (stylospores), and winter spores or 

 teleutospores ; out of the latter are formed sap- 

 rophytically on the fallen leaves a promy- 

 celium which develops sporidia ; these sporidia 

 get lodgment upon the barberry leaf in which 

 the aecidia develop and form besides spermo- 

 gonia, the so-called spermatia. We have there- 

 fore three or four parasitic forms and one free 

 form. Among the smuts or Ustilaginea the 

 mycelium develops spore-bearing filaments 

 upon the grain ; out of these spores a promy- 

 celium with sporidia is formed saprophytic- 

 ally ; the sporidia may invade young plants 

 and so begin again the parasitic cycle, but they 

 are able also to vegetate saprophytically for 

 countless generations in a torula form. Here 

 two saprophytic forms and one parasitic form 

 are able to exist. The spores of most vege- 

 table parasites are able to develop either a sap- 

 rophytic form or another parasitic form. 



An alternation of generations of just this 

 kind has not yet been demonstrated among dis- 

 ease-producing bacteria and other microbes, but 

 with some species its existence is not wholly 

 improbable. It is obvious that such complica- 



