of the Nuclei in Peronospora parasitica . 135 
this stage ; it appears to belong entirely to the nucleus, and it 
persists, so far as can be seen at present, during the whole of 
the changes which the nucleus undergoes during its division. 
The nucleus next elongates slightly, and at the same time the 
threads arrange themselves longitudinally in the equatorial 
plane. A separation of the threads then takes place into two 
groups which travel to either end of the nucleus (Fig. 1, 4 , 5 ). 
At this stage the nucleus appears to consist of an oval mass of 
slightly staining substance, in which the deeply stained threads 
are to be seen grouped at each end ; this mass probably con- 
sists of the nuclear membrane enclosing the nucleoplasm. 
The next stage is seen in the gradual constriction or inflection 
of the nuclear membrane (Fig. 1, 6 ) ; and at the same time 
the filaments at each end of the nucleus begin to unite with 
each other to form the daughter nuclei. A narrow bridge 
connecting the two nuclei remains for some time, but this is 
finally broken down, and the two daughter nuclei are com- 
pletely formed. 
I have not been able to observe the formation of a spindle- 
figure, nor the longitudinal splitting of the chromatic elements. 
The Nuclei of the Oogonium. 
The oogonia (Fig. 4, etc.) are formed as terminal or inter- 
calary swellings of the hyphae. Large quantities of pro- 
toplasm and numerous nuclei pass into them from the hyphae, 
and when they have reached their full size they are cut off 
from the thallus by one, or if their position is intercalary, 
by two transverse walls. About the same time that the 
oogonia are cut off, one or more antheridia are developed 
close to each oogonium (Figs. 4 and 5, a), either from the 
hypha in connection with it or from hyphae adjacent. The 
antheridia also contain protoplasm and nuclei derived from 
the hyphae, and like the oogonia are cut off from the thallus 
by transverse walls. The oogonia and antheridia are found 
in large numbers in all parts in the host-plant. They appear 
to be developed at first near the surface, but are afterwards 
