Pleodorina illinoiensis Kofoid in Britain. 181 
few days. The microgametes were about 20/x long ; they penetrated 
into the female colony, but the fertilisation was not seen. The 
megagamete surrounded itself with a membrane. Misled by a 
superficial resemblance, the author compares the phialea to the 
blastula of an animal organism, as does Janet ( 9 ) in Volvox. 
It is evident from the above that the Harborne specimens are 
identical with Kofoid’s P. illinoiensis. If it had been permissible 
to select features from, say, two colonies and combine them into a 
single figure, an almost exact parallel to Kofoid’s figure might have 
been obtained. This species has been previously found in Britain 
in a pond on Skipwith Common, North Yorkshire, and in several 
places in Scotland. For this information and the opportunity of 
examining the Skipwith specimens, which are identical with mine 
except in the absence of the posterior mamelons , I am indebted to 
Professor G. S. West, who has also given me help in connection 
with the literature of the subject, and in lending a mounted specimen 
of Pleodorina californica. 
But it is plain, on considering the whole matter, that the 
distinction between Pleodorina and Eudorina is a somewhat slender 
one, so far as it relates to the difference in shape of the colonies 
and in size among the cells in the same colony. The additional 
distinction alleged in the original diagnosis, viz,, the occurrence of 
two types of cells in Pleodorina, gonidial and vegetative, is seen to 
be not absolute, but only relative; the vegetative cells in P. illinoi¬ 
ensis can divide, but usually do so less frequently and at a later 
period than the gonidial cells. Instead of regarding Pleodorina as a 
distinct genus, we might speak, as even Kofoid suspected, rather of 
a Pleodorina-stSLte of Eudorina, though it is not clear what causes 
the latter to assume that state in preference to the normal form. 
The four anterior cells, which evidently possess the greater command 
over the movements of the ccenobium, may be regarded as giving up 
in part their function of reproduction, in order better to fulfil the 
former role. Thus, as has been often stated, Pleodorina is an 
advance on Eudorina in the direction of Volvox, where the distinc¬ 
tion between somatic and reproductive cells is strongly accentuated. 
But the first-named genus seems to be still in a state of flux* so 
that varying evolutionary stages between it and Eudorina can at 
times be met with. However, many phanerogamic genera rest 
upon no better basis, therefore, for the sake of convenience, 
Pleodorina should still be maintained until the facts are better known. 
The Botanical Laboratory, 
University, Birmingham. 
