Pythium de Baryannm, 659 
is a reduction in the number of chromosomes during the first 
mitosis.’ It is evident that further investigations on the 
division of sexual as well as vegetative nuclei are necessary, 
before any definite explanation can be given as to the signifi- 
cance of the nuclear division in the gametangia. 
Formation of the Oosphere and Fertilization. 
While the nuclei arrange themselves in the peripheral part 
of the oogonium, the central part of the protoplasm becomes 
more or less vacuolate. After the division of the nuclei, the 
entire protoplasmic contents differentiate into ooplasm and 
periplasm (Fig. 17 ). In Cystopus and Peronospora this differ- 
entiation of the protoplasm into two parts takes place a little 
earlier than in Pythium ; it occurs just about the time of 
nuclear division, or a little before. I have failed to observe 
any differentiated mass of protoplasm in the centre of the 
ooplasm, as found in all the species of Peronosporeae so far 
studied, and named by Stevens the coenocentrum. 
A single nucleus enters into the ooplasm and remains 
usually at the centre, while all the other nuclei are left in 
the periplasm and finally degenerate (Fig. 18 ). Now the 
oosphere with its nucleus is formed, but no wall seems yet to 
be formed around it. 
It is interesting to note here that in Cystopus Bliti , accord- 
ing to Stevens (’99), the oosphere is multinucleate, and the 
nuclei divide once more before they fuse with the nuclei from 
the antheridium ; the same state of things was found, by the 
recent investigations of Stevens (’ 01 ), in Cystopus Portulacae , 
whose oosphere was described by Berlese (’97) as uninucleate. 
Stevens (’01) also found that in Cystopus Tragopogonis , the 
oosphere is multinucleate and the nuclei divide again previous 
to fertilization, but only one of them becomes the functional 
nucleus of the oosphere, while all the other nuclei degenerate. 
This species is considered to be a connecting link between 
the above-mentioned two species of Cystopus and C. candidus 
whose oosphere is uninucleate from the beginning. Stevens’ 
recent researches on Cystopus candidus differ from those of 
X x 
