36 
Busk on Volvox glohator. 
between one sudden contraction and the next, and about half 
of it perhaps was taken up by the slow dilatation of the space. 
This contractile vacuole always reappears in precisely the same 
spot. It would seem to exist, or at all events to present a 
contractile property only for a limited period, and to disappear 
soon after the formation of the brown spot, when, as I con- 
ceive, the zoospore has reached its maturity. The most 
favourable cases in which this contractile space is to be sought 
for, are those in which the Volvox is in the most vigorous state, 
and especially in that variety in which, owing perhaps to the 
copious supply of nutritive matter, the amount of protoplasm 
is very abundant, and the zoospores consequently very numer- 
ous and connected to each other, not by slender filaments but 
by wide processes, as in figs. 26, 27, which latter shows a 
contractile space situated in the base of one of the connecting 
bands of protoplasm. With the exception of the small space 
occupied by this contractile spot, the zoospore at first appears 
to be quite solid, and no distinct wall can be perceived around 
the green matter, but it rapidly changes. Owing either to the 
expansion of the vacuole, above described, after it has lost its 
contractile property, or to the formation of others of a different 
nature, and also perhaps in some degree to the absorption or 
consumption of some of the colouring contents, the zoospore 
gradually becomes more and more transparent (fig. 7) ; till at 
last, the greater part of it is clear and colourless, and what 
remains of the green matter contracts into a small irregular 
mass, adherent to the bottom or sides of what is now a cell — 
primordial cell of Cohn. (Figs. 5, 6.) 
Each cell, when fully formed, usually presents a brown 
spot, which is adherent to one side of the cell towards the 
narrow end (figs. 5, 6) ; and what is remarkable, it will be 
noticed in a perfect specimen, that the brown spots are placed 
in a corresponding situation in all the cells, that is to say, all 
the cells appear to look the same way. This is the so-termed 
eye-spot of Ehrenberg. When examined with a high power 
(800 — 900 diam.) it presents the form of a cup or disc, con- 
cave on the side which looks outward, and convex on the 
other. Though placed quite on the side of the cell, and pro- 
jecting a little upon it, the brown spot is nevertheless always 
covered by a thin membranous expansion of protoplasm, or, in 
other words, it is always lodged within the substance of the 
zoospore. Though most usually present, the brown spot does 
not appear, in all cases, to be at any period a necessary con- 
stituent of the zoospore. It is one of the most persistent 
however, remaining visible as long as any portion of the zoo- 
spore is discernible. Besides the above-described elements, the 
