OOSPORES. 
277 
swells, and becomes closely attached to it; and the thicker part becoming separated 
by a septum (just as takes place with the oogonium itself), developes into an anthe- 
ridium. As soon as the oosphere is formed, a fine branch of the antheridium {B, an) 
reaches it, penetrating the membrane of the oogonium. After fertilisation the oosphere 
becomes surrounded by a coat which thickens and forms an external rough dark-brown 
exospore and an inner endospore. These oospores remain dormant through the 
winter and then germinate; in the case of Peronospora Valerianellce they form a myce- 
lium on moist ground ; those of Cystopus, however, produce zoospores ; the endo- 
spore (i) forces itself like a bladder out of the ruptured exospore (Fig. 181, F), and 
then bursting, the zoospores (G) are set free, which behave in exactly the same 
manner as the zoogonidia produced from the conidia of this genus. 
Fig, 181. — Cystopus Candidus. A mycelium with young oogonia ; B oogonium og with oosphere os and antheridium an ; 
C ripe oogonium ; D ripe oospore ; E, F, G formation of zoospores from oospores ; ? endospore (after De Bary, X 400). 
The genus Empusa, in which no sexual organs have as yet been discovered ^ is 
probably related to the Saprolegnieae and to the Peronosporeae. Empusa muscce is the 
parasite which proves fatal, more especially in the autumn, to house-flies. If these 
insects remain attached to the window-panes they become surrounded by a powdery 
substance which consists of extruded conidia. These conidia are capable of forming 
^ [Brefeld, Untersuch, über die Entwickelnng der Empusa Muscce und E. radicans, 1871. 
Nowakowski and Brefeld have both recently published observations on Empusa muscce {Entomoph- 
thoroL) in the Bot. Zeit, for 1877. While Nowakowski states that he has observed zygospores, 
Brefeld believes that the resting spores arise asexually. Nowakowski thinks the Etitomophthorece 
allied with Piptocephalidece.'] 
