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sac, and possessing a diameter of about 1-170”, already contains a
certain number of enlarged gonidia, destined in due time to become its
own progeny. Not only so, but long before its discharge, and while yet
it exists as a daughter-cell within t i e protecting cavity of the parent
generation, these selected gonidia are already visible as spots larger and
darker than their fellows. (Plate 22, Fig. 1.)
“ The history of these selected gonidia, as it may be traced in a
daughter-sphere recently cast forth to seek its fortunes in the world of
waters around it, is as follows :—The enlarged gonidium is at first a flat,
thill circular disc, appressed to the internal surface of the sphere, and
being surrounded by eight of the ordinary zoospores, is derived from
the coalescence of the two central ones out of a group of ten. (Plate
23, Figs. 1, la.) Shortly, this disc assumes a more distinctly oval form,
with a slight constriction across its lesser diameter, in which stage it
often much resembles a young Cosmarium. (Plate 23, Figs. 2, 2a.) I t
is soon seen to be clearly subdivided into four, and its thickness having
grown pari passu with its superficies, the group now protrudes into the
internal cavity of the parent-plaiit. (Plate 23, Pigs. 3, 3a.) Repeated
subdivision now goes on rapidly (Plate 23, Pigs. 4, 4a), till the whole
body assumes a sphærical form, a distinct cell wall being at the same
time formed, which is revealed by careful illumination, and still more
clearly by the use of reagents, as a hyaline sphere concentric to and of
larger diameter than the green one within it, so that there appears to be
a clear space or ring between the tw^o when seen in section. (Plate 22,
Fig. 5, 5a.) Finally, the young Volvox consists of a vast number of
deep green granules closely packed together, and by mutual pressure
driven to assume a more or less distinctly hexagonal form, and corresponding
in number to the gonidia which are to stud its surface when its
growth is completed. Shortly hereafter, the whole organism continuing
to increase in size, clear spaces appear between the gonidia, showing
that the enlargement of the cell tvall and its interspaces is outstripping
that of the gonidia,-which, are now approachiug maturity. The interlacing
connecting threads are developed simultaneously. (Plate 22, Pig.
la.) During the whole process the centre of the young Volvox spheres
continually recedes from the periphery of the parent, so that when the
group of young ones has attained the full development of which it is
capable in this stage they are often pretty cl 'sely packed in the internal
space, and sometimes even slightly deformed by mutual pressure, each
by this time closely resembling the parent in miniature, and already
containing enlarged gonidia of the third generation. (Plate 22, Fig. 1.)
By this time the clear space originally visible between the gonidia and
the cell wall has been obliterated, and the cilia may be seen protruding
through the latter. Some writers state that the daughter-cells rotate
at this period within the parent cavity. I have frequently seen them
oscillate so far in one direction and then back to their original position,
but have never observed a true rotatory motion. Finally, the young
Volvoces are liberated by the rupture of the parent sac, at a special
point, clearly marked out for this purpose in its structure. I have not
met with any observations on this point, but have fully convinced
myself that it may always be predicted at what point this rupture will
be effected.
“ The combined action of the pairs of cilia in which the gonidia
terminate is the actuating power whence proceed both the rotatory and
the progressive movement of Volvox, and these are both in a definite
direction. If an imaginary axis be drawn through the sphere, the progressive
motion being, so to speak, from the north to the south pole of
that axis, the rotatory motion is usually from west to east, though not
always, being occasionally reversed for a few seconds ; but for the
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greater part of the time it is regularly in the direction indicated, and
the point of rupture of the sphere luill be at its north pole.
“ I t is difficult to determine precisely how this rupture is accomplished,
but I believe it to be by a special contraction of the walls of the parent,
or of the invisible primordial utricle, not by the outward pressure of
the daughter spheres, this force being evidently inadequate to produce
the result where their number is small, whatever it may be when it
reaches its maximum.
“ Shortly before the emission of the young the cell commonly assumes a
slightly pyriform shape, and then slowly opens at its apex, but the aperture
is of less diameter than that of the young Volvoces, and as each of these
passes out, the month of the bag is visibly stretched, and resumes its
original size after each daughter sphere has escaped, so that it evidently
possesses considerable elasticity, a property also made manifest by the
fact that the normal form of Volvox may be considerably flattened by
the pressure of a glass cover, and yet resume both its sphærioal form and
its motion when this pressure is removed.
“ Moreover, the daughter sphere passes out without rotating, and from
whatever cause it derives its impulse, this often suffices to drive the
young Volvox clear of the month of the sac to a distance equal to
several times its own diameter, in wffiich position it pauses motionless
for some seconds, and then, commencing to rotate gently, sails away, at
first slowly, then more and more rapidly, to enjoy its independent
existence.
“ After the rupture of the sac, the gonidia near the edges of the
opening are seen to quiver, from the action of the cilia, where they are
partially freed from the support of the surrounding envelope, and the
same thing occurs when they are forcibly torn from their attachment,
in which case they may even move for awhile freely through the
water.
“ The general action of the cilia continues for some time, and the
empty sphere rotates as before, its general direction being still from
north to south, with the open end to the rear. After a time, which I
cannot specify, the cilia cease to play, and the organism decays, having
fulfilled its destiny in life.
“ The birth of the young Volvoces is affected by various circumstances.
Doubtless the process is, under natural conditions, most active in the
early hours about dawn, when the analogous functions of similar
organisms are well known to be most energetic, but in order to see the
phenomenon in full vigour it is only necessary to place a number of
mature parent-spheres, such as are found in every colony, in a shallow
live-trough, and to bring them into a warm room. In an hour’s time
almost all the young plants will have been liberated. Light and heat
stimulate the action, while cold and darkness retard it. The ciliary
action is affected in a remarkable degree by altered external conditions.
If a drop of water considerably colder than that in which the Volvoces
are floating be allowed to flow in under the cover-glass, the whole are
paralysed for some seconds, after which they slowly resume their motion.
A sudden mechanical shock produces a similar effect. A sufficient
degree of heat to make the water distinctly tepid to the feel causes
instant and simultaneous death of the whole colony.
“ During the day the majority of the Volvoces contained in a shallow
vessel rise to the surface, although they avoid strong direct sunshine,
while at night they retire in a cloud to the bottom.
“ The astonishing number in which the spheres at times appear in
some pool, and their equally sudden disappearance, have been frequently
remarked. Doubtless a very slight change in external conditions suffices
on the one hand to favour the development of countless thousands of
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