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 (jB, 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. \%i.— Cystopus candidiis. A mycelium with young: oogonia ; B oogonium og with oosphere os and antheridium «« ; 

 C ripe oogonium ; D ripe oospore ; E, F, G formation of zoospores from oospores ; i 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 musccB 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 



|H| substance which consists of extruded conidia. These conidia are capable of forming 



^B 1 [Brefeld, Untersuch. iiber die Entwickelung der Empusa Mtiscce uud E. radicans, 1871. 

 I^Wowakowski and Brefeld have both recently published observations on Empusa musc<2 (Entomoph- 

 ^mkora) in the Bet. Zeit. for 1877. While Nowakowski states that he has observed zygospores, 

 ■Wfirefeld believes that the resting spores arise asexually. Nowakowski thinks the Entomophthorea 

 allied with PiptocepkalideiE.] 



