f
ruptured sphere, and seems to show that there is a denser layer of
thick matter, whatever its nature may be, disposed in a somewhat
regular manner, being concentrated near the south pole of the axis of
rotation, whence it spreads over the inner surface in streaks resembling
the lines of longitude on a terrestrial globe.
“ Both from its position and from the rapidity with which it is stained
by aniline purple, without which its existence is apparently absolutely
undemonstrable—(in which respect it is in marked contrast to the outer
cell-wall, which latter is only faintly tinted by somewhat prolonged
application of the reagent, and then only where the hexagonal structure
exists)—I have no doubt that this inner layer is the true ‘ primordia
utricle ’ of the cell, and possesses that character of vital and formative
matter which distinguishes this element of cell-structure from the outer
wall, which, on the other hand, probably consists of cellulose or some
similar compound. Probably the ai’rangemeut of this inner layer, in
radiating lines or ribs, contributes to the elasticity of the fabric, whereby
it is enabled to open at a given point for the escape of the young, and
to contract again after their emission.
“ The increase of individuals by the means already described is
strictly an instance of subdivision.
“ But Volvox glohator also affords an instance of true alternation of
generations. As may probably be affirmed of all living organisms, its
life-history would be incomplete without a process of sexual reproduction,
and accordingly, after a long sequence of asexual generations, a
strictly sexual process intervenes, from wffiich result certain spores
destined to lie dormant for a w'hile, and, like the zygospores of the
Conjugate Algse, to resist vicissitudes of condition and climate
through the rigours of winter, and then to produce the parent form in
the succeeding year, when external conditions again favour its develop-
ment. ^ j
“ Cohn fully traced the various stages of this process, and described
them in the ‘ Beiträge zur Biologie der Pflanzen ’ (1875, Vol. 1., Heft. 3),
and in the ‘Annales des Sciences Naturelles ’ (4 ième Ser. Bot., Tom.
V., 323) ; and his observations have been more or less confirmed by
other investigators, especially by Carter (Ann. Nat. Hist., 3rd Ser., Vol.
III., 1859, p. 1), and more recently, in 1877, by a French botanist, M.
F. Henneguay.
“ Cohn and Carter both hold that there are two varieties of Volvox,*
one monoecious, the other dioecious, and the latter maintains that
Sphoerosira Volvox is the male form of the dioecious sub-species.^ Be
that as it may, the reproductive process in the moncecioua form is as
follows :—The sexual reproductive cells, male and female, occur in
spheres of unusual size in the autumn, and are few in proportion to the
number of sterile cells, and the reproductive process does not occur
simultaneously with, but-as a climax to a long series of asexual generations.
On tbeir first appearance the gynogonidia or female cells are
about three times the size of the sterile ones, of a deep green colour,
and of a frothy consistency from abundance of vacuoles. They are
easily distinguished from the parthenogonidia by their never subdividing.
(Plate 22, Pig. 5&.) They next become flask-shaped, their
narrow end touching the periphery of the sphere, and the broader end
hanging free in the internal cavity. (Plate 22, Fig. Finally, they
assume a sphærical form, and become oospheres, each enveloped in a
gelatinons membrane. (Plate 22, Fig. 5Z>^,
“ The androgonidia, or male cells, at first c l o s e l y resemble the parthenogonidia,
but undergoing division in two instead of three directions,
* The two forms are here accepted, after Stein, as Volvox glohator and Volvox
minor.
develop into plates or discs of cells, not into spheres, and ultimately
resolve themselves into bundles of naked elongated cells, in which the
chlorophyll is transformed into a reddish pigment, each with a long
colourless beak, with a red ‘ eye-spot ’ and two cilia. (Plate 22, Fig.
5a, a2.) About the same time that the oosphere is mature these
antheridia begin to move from the combined action of their cilia (Plate
23, Fig. 10), and then break up into separate antherozoids, which finally
become free, and move rapidly within the cavity of the sphere. (Plate
23, Fig. 5a^.) Assembling round the oospheres, they penetrate the
envelopes of the latter (Plate 22, Pig. 4), coalesce with their contents,
and the oosphere, thus fertilised, becomes an oospore, which soon develops
a cell-wall covered with conical stellate projections, and a second smooth
internal membrane. ( Plate 23, Fig. 11.) The chlorophyll now gradually
disappears, and is replaced by an orange red pigment. In this condition
the oospore constitutes the Volvox stellatus of Ehrenberg. I t is liberated
by the decay of the parent.cell, and sinks to the bottom of the water
to hibernate. The subsequent history of these bodies has been traced
by Cienkowski, and more recently by Henneguay (“ Journal de
Micrographie,” Vol. II., p. 485, Bull. Soc. Philomath, Pans, July, 1878).
“ Cohn believed that they must be dried up before germination was
possible. Henneguay has now observed that this is not so. In spring
the outer case of the spore (exospore) is ruptured, and the swollen contents
(endospore) project throngh the opening. The contents then
divide gradually into two, four, eight, sixteen, or more small cells, which
become bright green, each meanwhile acquiring two vibratile cilia while
still contained within the inner membrane of the spore. The cells, at
first in close apposition, separate further from one another by interposition
of gelatinous hyaline matter, the outer membrane disappears, the
cilia become active, and the young Volvox, already containing some
elements larger than the others, and destined, in due course, to produce
daughter-spheres, moves freely through the water. ‘The spores of
Volvox, therefore, germinate in water, and each of them produces a
single colony by a process of segmentation identical with that which
gives rise to a daughter-colony at the expense of a cell of the mother-
colony.’
“ The sequence of asexual generations is repeated for many months,
and in the following autumn the alternation of generations is again
completed by the intervention of the processes just described.
V o lv o x globator. Linn. Syst. Ed. x.
Larger coenobia, with very nnmerous cells (12,000), always
with daiighter-coenobia enclosed within the mother, evolved
without sexuality; fructification dioecious ; the male coenobia
nourishing numerous red fascicles of spermatozoa ; the female
coenobia originating 20-40 sexual cells, which after fecundation
are resolved into as many red globose oospores, surrounded by a
hyaline stellate epispore (= V o lv o x stellatus, E h r.).
S i z e . C æ n o b ium a s m u c h a s 1 m m . d iam .
Ehrb. Infus. 68, t. 4. Dujardin Zoophy. 312, iii. f. 25. Stein
Infus. p. 46. Eabh. Alg. E u r. iii. 97. Pritchard Infus. 526,
t. 20, f. 32-47. Busk. Trans. Mior. Soc. 1853, p. 31. Williamson
Trans. Micr. Soo. 1858, p. 45. Ourrey Ann. Nat. Hist.
1859, p. 5. Dr. J . B. Hicks in Micro. Journ. 1861, p. 281 ; in
1%
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