Cytology of Pythium ultimum , n. sp. 271 
of species of the same genus (as at present defined) may occur, 
as has been well shown recently in the case of Cystopus. 
It would be interesting to know the exact meaning attached 
by Fisch to ‘ verschwindet ’ in the last line of the quotation. 
For nuclear fusion he uses the verb ‘ verschmelzen,’ which is 
the usual term. It is noteworthy that in bad preparations 
the male and female nuclei are very difficult to demonstrate. 
They are apt to disappear from sight altogether. It is possible 
that Fisch never really traced in Pythium the nuclear fusion 
characteristic of fertilization. It is certain that in one species 
of Pythium at least no such fusions as those described by 
him take place. 
Dangeard (’92) says : ‘ Le Pythium proliferum poss£de dgale- 
ment de nombreux noyaux, soit dans les oogones, soit dans 
les antheridies/ The observations recorded in this com- 
munication show that Dangeard’s view, which so far as it 
goes does not materially differ from that of Fisch, is a 
correct one. 
My work on Saprolegnia and Achlya , , especially that part 
of it which dealt with the mode of fertilization and the 
degeneration of nuclei, made me anxious to study some 
member of the Oomycetes in which there was no question 
as to the actual sexuality, and where moreover one could 
follow the course of degeneration in indubitably degenerate 
nuclei. The Peronosporaceae offered the right kind of material. 
Functional sex is universally admitted to be characteristic 
of this group. The periplasm might be expected to furnish 
the degenerate nuclei. That the genus Pythium is, as Fischer 
(’92) points out, a difficult one, with a considerable number 
of ‘ critical ’ species, in part at least, very badly defined ; that 
nevertheless it has been selected to furnish type plants in two 
well-known and highly esteemed textbooks ; that its study 
has been specifically introduced into more than one syllabus 
of elementary botany (or biology) ; and above all, as already 
pointed out, that a detailed knowledge of its karyology was 
still a great desideratum — furnished ample motives for its 
selection as the subject of a thorough investigation. 
