306 Trow . — Observations on the Biology and 
altogether and not simply their nuclei. In this way the 
periplasm probably arose. There is nothing very remarkable 
in the fact that the male and female nuclei in Cystopus Bliti 
fuse in pairs, for this propensity they have no doubt inherited 
from their algal ancestors. We have now only to go a step 
further and imagine the habit of the nuclei of monopolizing 
the cytoplasm of their neighbours (as already seen in Sapro- 
legnia ), still further intensified to see how the uninucleate egg 
might arise in such forms as Cystopus candidus , Peronospora 
parasitica , and Pythium ultimum. There are many missing 
links in this chain of evidence no doubt, but perhaps one 
gap may be filled up by considering the behaviour of the 
nuclei in the differentiation of the oosphere in Pythium 
ultimum . More than one nucleus frequently remains behind 
in the egg to undergo division there. There is in this, at 
any rate, a suggestion of an ancestral condition in which 
numerous nuclei were present in the oosphere. 
It is now apparent that the uninucleate egg of Pythium 
ultimum and its allies, is the homologue of the multinucleate 
one of Cystopus Bliti — in the terminology of Stevens is a 
‘compound oosphere.’ In view of this relation it is almost 
necessary for Stevens to withdraw or amend his terminology. 
It certainly seems rather absurd to describe the uninucleate 
oosphere of Pythium as compound. 
It is obvious that the uninucleated egg might have been 
evolved in other ways, but this view gives us a provisional 
explanation of the periplasm which is not wholly theoretical, 
whilst limiting our speculations to the Oomycetes themselves. 
The cytology of the Zygomycetes is so obscure at present, 
as Davis has well said ; and the forms mentioned by him, 
in his attempt to create a phylogeny for Cystopus Bliti, 
show such remote affinity to the Peronosporaceae, that I feel 
it would be a mistake at present to direct attention to the 
Zygomycetes with a view to harmonizing these two types 
of fertilization. There certainly appears, contrary to the view 
propounded by Davis, to be no difficulty in connecting 
Cystopus Bliti, through forms like Cystopus candidus , Pythium , 
