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parent globe. This process was observed to oocnpy about twelve hours.
Cohn also observed the division of each of the eight primordial cells into
a gi-eat number of microgonidia, which swarm within the globe, and
escape from it. Under certain circumstances each of the eight cells
secretes a cellular covering, and swims about in the interior of the
globe in the form of free Chlamydococcus-\ike cells. Eventually they
escape either by fissure of the globe or by its gradual dissolution, lose
their cilia, form a thicker membrane, become motionless, and accumulate
at the bottom of the vessel. If the vessel be permitted to become
dry, and afterwards filled with water, motile 8tephanos:phcBra reappear,
from which it seems probable that the green globes are the resting cells.
The resting cells vary much in size, and it is supposed that they
gro-w considerably after attaining a state of rest. The colonr is deep
green, sometimes yellowish or olive, and they possess a nucleus.
The dried resting spores absorb water, and their contents gradually
fill up the cavity of the containing membrane, and become cloudy and
granular ; the border becomes yellowish, and the red colouring matter is
contracted in the centre. The cells then begin to divide and pass
through successive stages, as shown on the plate (Plate 28, figs. 13
to 17). The four daughter-cells begin to quiver, and endeavour to
separate from each other. Two cilia are now apparent at the pointed
extremity of each of the four cells (fig. 19), by the action of which the
group begins to move as a whole; utlimately all trace of the enveloping
membrane disappears, and one by one the daughter-cells escape and
become free. At the moment of escape their diameter never exceeds
•01 mm., but they soon enlarge to a diameter of '013 to *015 mm.
The length of time which elapsed between the immersion of the
dried resting spores and the first appearance of the motile cells varied
from nine to twenty-four hours. I t was observed that those resting
spores which did not produce zoospores within six days never did so
afterwards, although they continued to live, and seemed perfectly
healthy. Zoospores produced in November did not advance beyond the
first stage. Others produced in March remained only a few hours in
that condition, after which time a delicate membrane was formed
round the body of the primordial cell; this membrane was at first
closely attached to the cell, but became gradually enlarged, by absorption
of water, into a colourless enveloping vesicle, usually globular,
but sometimes oval, having two openings, through which the cilia
penetrate. In this condition they attain a diameter of '017 to *022 mm.,
and are not distinguishable from encysted forms of Chlamydococcus
pluvialis. Other zoospores, produced in April, attained a larger size,
and the protoplasm of the primordial cell, instead of retaining its continuous
outline, became elongated here and there into simple or forked
mucilaginous rays, which were either colourless or green from the presence
of chlorophyll. These rays are probably produced by the protoplasm adhering
at certain points to the surrounding membrane, and being carried
outwards by its growth. The Chlamydococcus-like form only lasted a
few hours ; towards the evening the zoospores mostly began to divide.
The rays are drawn in, the primordial cell becomes round, it then elongates,
is constricted, and ultimately is divided into four quadrants.
These are again bisected, and eventually eight wedge-shaped portions are
formed, whose contour lines, like the spokes of a wheel, meet in the
middle.
Here is to be noted a distinction between the StepJianosphwra and
Chlamydococcus; for while in the latter the individual portions of a
primordial cell separate entirely from each other, each developing its own
enveloping membrane, and ultimately escaping as a unicellular individual,
in the former the eight portions remain united as a family. The
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coloured contents of the individual portions become drawn back towards
the periphery in centrifugal direction, a colourless plasma remaining
about the central point ; this disappears at first in the centre, a cavity is
formed in the middle of the disc, and as tliis enlarges the eight portions
assume the form of a wreath, consisting of eigiit globular or ellipsoid
bodies in close contact, usually not exactly in one plane, owing to the
outer membrane not having expanded in proportion to the enlargement of
the plasma. The original cilia continue active, causing tlie motion of
the whole organism, until the eight portions are completely individualized,
and then their motion ceases. The separate parts of the plasma now form
eight independent but closely packed niembraneless primordial ceils.
Mjortly afterwards a delicate membrane common to them all is secreted
heneathttie mother-oell membrane round the disc formed by the primordial
cells. This membrane is at first in contact, but afterwards becomes further
and further removed as it swells and tends to assume a globular form. By
the motion of the cilia the mother-cell membrane is thrown off, and the
young family escapes into the water. When the Ghlamydococcus-like
unicellular Siepliwnosphaera has commenced its (ffvision early in the
evening, the division into eight is perfected during tlie night, and early
in the morning the young family quits its cast off iiiother-cell membrane.
In the course of the day the individual primordial cells and their
common investing membrane grow until the latter attains a diameter
cf -04 to -048 mm. During this growth the shape of the primordial
cells is changed by the formation of various prolongations, but in the
course of the afternoon they again become round, and during the evening
division commences in them precisely similar to the process in the
nnicellular Stepihanosphoera. On the following morning we find eight
young families. I t is calculated that in eight days, under favourable
circumstances, 16,777,216 families may be formed from one resting cell
of StephaHosphæra.
We have given bnt a barren outline of the history of this little plant,
but for further information must refer the student to Cohn’s Original
Memoir in Siehold and Kolliker’s Zeitsohrift fur Zoologie, 1852, p.
77. Translated in the “ Annals of Natural History,” 2nd series. Vol. X..
pp. 321 and 401. Also Cohn and Wiohura’s subsequent inemoir “ Ueber
Stephanosphæra ” in Nova Acta Acad. Leop. Car., 1857, part I., Vol.
XXVI. Of which an abstract is given by Currey in “ Quarterly Journal
of Microscopical Science,” Vol. VI. (1858), p. 131. Also an admirable
smnmary by Archer in the “ Quart. Journ. of Microscopical Science ”
for 1865, p. 117, with additional observations by himself.
Plate X X V IIl. fig. 1. I’olar view of family with globose primordial
cells ; 2, equatorial view, with fusiform primordial cells ending in
mucous filaments ; 3, the same, with primordial cells collected on one
side ; t , commencement of formation of macrogonidia ; 6, all eight primordial
cells divided in fours ; 6, division advancing so that each
primordial cell consists of eight cuneate segments ; 7, further advanced
in division, movement in the mother-oell having commenced; 8,
division completed, eight young individuals revolving in the motlier-
cell ; 9, the eight original primordial cells broken up into microgonidia;
10, young Stephanosphæra, (figs. 1 to 10 X 300) ; 11, microgonidia after
exit from mothsr-cell X 500 ; 12, full-grown resting cells ; 13, commencement
of division in resting cell ; 14 to 19, successive stages lu division
of resting cell ; 20 to 22, naked zoospores ; 23, 24, encysted zoospores ;
25 to 27, division of encysted zoospores ; 28, yonng eight-celled family
resulting from division of encysted zoospore; 29, 30, young families,
all after Cohn X 400 ; 31, amæboid condition of primordial cells, after
Archer.
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