of the Nuclei in Peronospora parasitica. 143 
staining nuclei, which in very thin sections, obtained by means 
of a ribbon-section-cutting microtome, and stained in a very 
dilute solution of Kleinenberg’s haematoxylin, exhibit a very 
distinct nuclear structure. 
The division of the nucleus takes place by a process of 
karyokinesis, comparable to that which takes place in the 
division of the nucleus in the higher plants. This can be 
most satisfactorily observed in the nuclei of the oogonia. 
The nuclei of the oogonium at an early stage in the develop- 
ment of the latter are spherical or slightly oval, vesicular 
bodies, each of which contains a large mass of chromatin, 
forming a peripheral layer on its wall. 
All the nuclei of the oogonium divide, and the process of 
division is accompanied by changes in the protoplasm, leading 
to the formation of the oosphere. These changes are more 
complicated than is generally supposed. 
The protoplasm of the oogonium at an early stage appears 
to be a homogeneous, granular mass, containing numerous 
nuclei, as described above. 
Numerous vacuoles appear in the centre of the oogonium, 
causing the greater part of the protoplasm and all the nuclei 
to be restricted to the periphery. At the same time the nuclei 
swell up, and exhibit a thread-like structure. They become 
arranged very regularly, and form a single layer in the parietal 
protoplasm. 
The chromatic threads next arrange themselves in the 
equatorial plane of the nucleus, and then divide into two 
groups of threads, each of which forms a daughter nucleus. 
The daughter nuclei again divide, and then two or perhaps 
more pass towards the centre of the oogonium, and soon after- 
wards the cell-wall of the oosphere begins to form on the inner 
side of the parietal layer of protoplasm, leaving this, together 
with the remainder of the nuclei outside, to form the peri- 
plasm. 
From this mass of protoplasm and nuclei both the endo- 
sporium and the exosporium are formed. One or more 
antheridia are developed in connection with the oogonium. 
