of the Nuclei in Peronospora parasitica. 129 
which take place in the nuclei of the higher plants are in- 
cluded. It is a recognised feature of all karyokinesis that the 
deeply-staining substance — chromatin — is capable of becoming 
re-arranged in such a manner as to allow of an equal or in 
some cases perhaps even an unequal division of it into two 
parts, each of which becomes a perfect nucleus. The simplest 
expression of these changes in the division of ordinary nuclei 
is given by Boveri 1 , who states that it consists in ‘ the col- 
lection of the chromatic nuclear material in a (definite) number 
of isolated parts, of characteristic form, varying with the kind 
of cell — the chromatic elements ; the formation of an achromatic 
figure of threads either from the nucleus or from the cell-sub- 
stance, with two poles ; the arrangement of the chromatic 
elements so far as their number, size, and form allow, in the 
equatorial plane of the achromatic figure ; the division of the 
chromatic elements into two halves of which one half goes 
to either pole ; the transformation of the daughter ele- 
ments into the network of two new nuclei.’ The c chromatic 
elements’ are now known to consist only in part of chromatic 
substance. 
So far as I am aware two observers only have made re- 
searches upon karyokinesis in the nuclei of the fungi. Rosen- 
vinge 2 has obtained indications of karyokinesis in the basidia 
of certain Agaricineae, but was not able to demonstrate it 
satisfactorily; and Hartog 3 has recently shown that some 
phenomena of karyokinesis occur in the division of the nuclei 
in the Saprolegnieae. The object of this paper is to show 
that the nuclei in the cells of Peronospora parasitica have 
a definite structure ; and that the process of division is a 
karyokinetic one closely allied to that which takes place in 
the nuclei of the higher plants ; and also that the nucleus 
plays an important part in the formation of the oospore. 
1 See Waldeyer, Ueber Karyokinese und ihre Beziehungen zu den Befruchtungs- 
vorgangen. Archiv f. Mikr. Anat. 1888. See also the translation by Dr. Benharn 
in Q. J. M. S., July 1889. 
2 Ann. des Sci. Nat., serie vii, tome iii 
3 Recherches sur la structure des Saprolegniees. 
K 
