136 Wager. — Observations on the Structure ■ 
produced abundantly in the deeper tissues of the plant. 
They sometimes vary considerably in size, but as a rule 
the oogonia obtained in sections from the same piece of 
infected tissue do not exhibit much variation in this respect. 
The number of nuclei present in a single, young oogonium 
is considerable. I have counted as many as 1 12 ; and it 
is possible that in some oogonia there are more than this ; 
while in others the number is considerably less. The an- 
theridia contain from six to twelve nuclei. It is almost, if 
not quite, impossible to count the nuclei in an oogonium in 
a section cut by hand. The protoplasm of the oogonium 
is too dense and stains too deeply to allow all the nuclei 
to be seen, and it is impossible to obtain successive sections 
of an oogonium when cut by hand. In sections cut by the 
ribbon-microtome, however, these objections are not met 
with, and in a good series of sections we are able to obtain 
several successive sections of one oogonium, in each of which 
the nuclei can be easily seen and counted. The total number 
of nuclei present in a single oogonium can then be obtained by 
taking the sum of the nuclei found in all the sections of it. 
The structure of the nuclei in the oogonium, at the time 
when the latter is delimited from the thallus, is similar to the 
structure of the resting nuclei in the mycelium. This simple 
structure is not retained long. During the formation of 
the oosphere the nuclei undergo changes which lead to 
their division ; the process of formation of the oosphere being 
a more complicated one than is generally supposed, at least 
so far as Peronosp or a parasitica is concerned. The prevalent 
ideas concerning the formation of the oosphere are somewhat 
as follows. The oosphere begins to be formed soon after 
the delimitation of the oogonium from the thallus by a slowly 
progressing separation of the protoplasm into a central denser 
portion which forms the oosphere, a smooth globular body 
surrounded by a delicate hyaline membrane ; and an outer 
portion, the periplasm, which surrounds the oosphere and 
takes part in the formation of the exosporium. Further 
changes, possibly connected with an act of fertilisation, then 
