202 MICRO-ORGANISMS AND DISEASE. [CHAP. 



differentiates into a number of spherical masses, the oospores, 

 which become invested with a membrane. These become 

 free and then germinate and grow into a mycelium. 



FIG. 119. SAPROLEGNIA OF SALMON DISEASE. 



A sporangium filled with zoospores ; in connection with them several young 

 mycelial threads. 



Saprolegnia grows on the skin of living fish, and causes 

 here severe illness often terminating in death. Thus the 



