82 



opening is formed between the oosphere and the antheridial or 

 fertilising tube, and it is probable that when ripe a small 

 quantity of protoplasm passes over into the oosphere from the 

 antheridiura. This passage of protoplasm has not been seen in 

 P. parasitica directly nor in any of the Peronosporas, but in 

 some members of the group, such as Pythium, a considerable 

 quantity of protoplasm has been seen to pass over into the 

 oosphere from the male organ. After fertilisation has taken 

 place, a thick outer cell wall begins to form around the oosphere, 

 and this is probably developed entirely from the periplasm. 

 This wall forms a thick, brown, coarse outer layer for the 

 oospore, fitting closely to its inner and more delicate one ; its 

 outer surface is covered with- irregular thickenings, giving it 

 a very rugged appearance. 



This description may be looked upon as a simple expression 

 of the phenomena of oospore formation. It does not, however, 

 represent all that takes place. The formation of the oosphere is 

 preceded and accompanied by changes wliich take place in the 

 nuclei of both the antheridium and oogonium. The structure of 

 these nuclei just at the time when the oogonium is delimited 

 from the filament is similar to the structure of the resting 

 nuclei in the mycelium. As development proceeds they become 

 much enlarged, and, at the same time, the chromatin becomes 

 distinctly visible in the form of threads (Figs. III. and VII.). 

 The chromatin itself appears to increase in amount, as there is 

 certainly more chromatin in the enlarged nuclei than in those 

 in the resting stage. Where this cliromatin is obtained is at 

 present doubtful. It may perhaps be absorbed in some way 

 from the surrounding protoplasm, but further investigation on 

 this point is needed. 



While the nuclei are increasing in size, changes are taking 

 place in the protoplasm. Numerous vacuoles appear in it (Fig. 

 VII.), and these gradually increase in size until at a slightly later 

 stage nearly the whole of the central space is taken up by them 

 (Fig. VIII.). During their formation the protoplasm is displaced 

 and becomes almost entirely restricted to the periphery of the 

 oot^onium, a small quantity only of granular protoplasm remaining 

 in the centre and connected with the peripheral portion by a few 

 thick protoplasmic strands (Fig. VIII.). The nuclei also accompany 

 the protoplasm to the periphery, the last few passing along the 



