Cytology of a new variety of A chlya americana . 137 
Achlya americana may be regarded, as Humphrey points 
out, as the middle member of a group of three species — the 
prolifera group — all of which have eccentric oospores and 
a number of other characters in common. The members 
of the group are very easy to discriminate, for Achlya prolifera 
is a diclinous species with pitted oogonia, and A. Dc Baryana 
(Humphrey) = A. polyandra (De Bary) is an androgynous 
species whose oogonia are destitute of pits. The affinities 
are roughly expressed by regarding A. americana either as 
a pitted A. De Baryana or an androgynous A. prolifera. 
The specific type as described and figured by Humphrey 
has obviously rather indistinct pits ; the form I am familiar 
with has pits almost, if not quite, as well-defined as those 
of the Ferax-grou^ of species of Saprolegnia. The American 
plants, too, apparently differ from those which I have examined 
in the greater number of antheridial branches and antheridia 
supplied to each oogonium, the colour of the wall of the 
oogonium, the structure of the oospore wall, and the size 
of the oospores. Other noteworthy points of difference 
have been indicated by italics in the description of the 
variety. 
Having maintained this variety, in a series of pure cultures, 
under regular observation for a period of one and a half years, 
I have become intimately acquainted with its life-history. 
The whole course of development has been followed macro- 
scopically dozens of times, and microscopically on several 
occasions. When the cytological work had been nearly com- 
pleted, it appeared advisable, and even necessary, to follow 
the development, step by step, with great care, so as to be 
able to correlate as rigidly as possible the observations made 
on stained sections with those made on the living material. 
In consequence of these studies, notwithstanding the attention 
paid to the genus by previous observers, and especially by 
De Bary, I am able to make a number of additions to our 
knowledge of its biology. 
