OOMYCETES 



67 



until only a thin protoplasmic wall layer with few nuclei remains (Fig. 

 40, A). The nuclei then undergo simultaneously a single mitotic 

 division and degenerate; finally the protoplasm splits, rolls up around 

 each of the remaining nuclei and forms uninucleate eggs, oospheres (Fig. 

 40, D). At first the egg nucleus shows a rostrate process with a definite 

 centrosome. 



<^d 



Fig. 40. — Saprolegnia monoica. Egg development. A, young oogonium with periph- 

 eral protoplasm; B to D, sections of the peripheral layer in different stages of develop- 

 ment; c, normal nucleus with centrosomes; d, degenerating nuclei. (After Claussen, 1908.) 



Meanwhile the antheridium has approached the oogonium and 

 partially surrounded it. About simultaneously with that of the oogo- 

 nium, the nuclei divide once. Hereupon the antheridia pierce the oogonial 

 membrane through pits. Within the oogonium the process divides, into 

 several branches, each of which approaches an egg cell and discharges a 

 nucleus into it. Nuclear fusion follows immediately. The fertilized 

 egg surrounds itself with a two-layered, smooth, colorless membrane: 

 it becomes an oospore with the character of a hypnospore. 



