Pleodorina illinoiensis Kofoid in Britain. 173 
colour was very brilliant; each eye-spot was circular in face-view, 
but subreniform or lunate ( i.e really basin-shaped), with the 
concavity directed outwards, in profile ; it would act like a concavo- 
convex lens. In the anterior cells the two ends of the crescent 
frequently protruded the cell-wall beyond the limit of the circular 
cell-outline. 
The colonies were multiplying rapidly at the time of gathering 
and continued to do so in the basin in which they were kept; the 
divisions took place chiefly in the night and forenoon. The process 
was as described by Conrad for Eudorina elegnns, except as to the 
last two states, G and H in his Pig. 9 ( 6, p. 334); for these the 
figure given here (Fig. 7) should be substituted. From the four- 
celled stage onwards the disc formed by the dividing cells is seen to 
be hollowed like a saucer, with the concavity directed outwards; 
this is called the plakea stage. Then the four corners rise up and 
meet in the 32-celled stage at a point above the centre of the 
plakea and at some distance from it, while the intervening cells 
arrange themselves in three alternating rows, as shown in Fig. 1, 
round the circumference of the ellipsoid. 
A hollow structure formed in this way is called a phialea; the 
centre of the plakea becomes the anterior pole, the opposite pole 
where the corner cells are united by mucus is called the phialopore. 
The globular or ellipsoidal colonies of Volvox are formed in a 
similar way, and it has been stated that in that genus the extrusion 
of the daughter-colonies always takes place by the rupture of the 
phialopore ; that may or may not be true, but it is certainly not so 
in the present species. 
When the phialea is first formed, the cells, which measure 
about 4/z, are in close contact and are all alike except one. For, as 
the division takes place, the original eye-spot remains unchanged 
and becomes a part of the cell in which it happens to lie ; but since 
it is always on the outside edge of the mother-cell, it finally (while 
still bright red) comes to lie in one, the first (Fig. 7) or more 
commonly the second, of the cells forming an outer edge of the 
plakea, i.e., usually in the first or second (or rarely in the third) 
zone of the phialea, counting from the phialopore. No other eye- 
spots could be detected in the just-formed phialea; these like the 
pyrenoids appear to be formed de novo, while the original eye-spot 
disappears, first losing its brilliance (though not its size) and becom¬ 
ing yellowish-red. The minute new eye-spots also seemed at first 
to be yellowish and more like an oil-drop ; they did not assume the 
