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young plants, and on the other, either to destroy the vitality of the
whole colony or to drive it to seek refnge in deeper water.
“ A curious instance of this sensitiveness to varying conditions of light
and heat occurred to myself. I had two shallow vessels in a north
window, each containing a goodly supply of Volvox. Cold and inclement
weather, which prevailed for weeks together, seemed to check their
increase, for I found but few young spheres from day to day among the
older ones. Thinking that a moderate degree of warmth would tend
to increase my colony, I transferred one vessel, fortunately not both, to
the floor of a warm greenhouse. In forty-eight hours all were dead, and
in a few days scarcely a vestige remained of the countless corpses which
had copiously strewed the bottom of the glass.
“ We must now revert to the minute structure of the mature parent-
sphere, which has been exhaustively studied by Cohn, Busk, and
Williamson.
“ In the outset it should be stated that the last-named observer
believes that there are two distinct forms of Volvox, in one of which
the peculiar structure which I am about to describe exists, while it is
absent from the other. Busk disputed the accuracy of Williamson’s
observations on this point, but in an appendix published subsequent to
the body of his essay he states that he has detected this same structure
in specimens from Manchester, but not in his own.
“ I have failed to develop it by the means recommended by Williamson,
but have succeeded in making it evident enough iu a great number
of specimens from Sutton, by the use of these reagents, and especially
by the application of aniline purple, an invaluable auxiliary in the
examination of minute vegetable cell-structures.
“ This substance stains the protoplasmic elements of such structures
to a colonr which appears deep purple by direct light and crimson by
dark background illumination, and reveals details which are wholly
invisible without its use.
“ The colour is, however, greedily absorbed by some of the materials
used by the microscopist, so that a judicious choice of these is necessary
to ensure success. Objects stained in this manner are, for instance,
rapidly bleached if mounted in gold-size cells, and I have for the present
adopted zinc-white in its place. Among other reagents which I have
used are eosin, iodine, iodised glycerine, carmine solution, potassium
permanganate, nitrate of silver, and other salts, some of which bring
into view various parts of the minute structure of p lan ts; but aniline
coloars, applied with due precautions, produce the most rapid and
striking effect.
“ Professor Williamson describes the structure in question as a network
of lines dividing the whole surface into hexagons, in the centre of
each of which is seated one of the gonidia.
“ The delicate ‘ protoplasm-threads ’ proceeding from each of these
to its six surrounding neighbours never pass throngh the angles of the
hexagons, but always through the side of each hexagon to the next
gonidium. (Plate 23, Fig. 3.) Hence it appears that ‘ the points of
adhesion are chosen prior to the development of the outer cell
membrane,’ in which light Williamson regards the hexagonal division.
In his specimens this structure was developed by immersion in glycerine
for some time. I have failed to obtain more than the faintest suggestion
of it by these means, but it is often brought out by the application
of aniline purple, as is also an important detail shown in drawings
made from his preparations, viz., that at the angles of the contiguous
hexagons there is sometimes a distinct doubling or separation of the
lines, whence he concludes that each side of the figure is really formed
by two delicate cell-walls in close juxtaposition, the duality of which is
only made evident by the action of reagents. (Plate 22, Fig. 2.) He
regards the globe of Volvox as a ‘ hollow vesicle, the walls of which
consist of numerous angular cells filled with green endochrome, &c., the
intercellular spaces being more or less transparent,’ and the ciliated
zoospore as representing the endochrome of a cell having two walls, the
internal one being separated from, the outer cell-wall, except at a few
points where it is retained in contact by the connecting filaments, and
the external one forming the hexagonal divisions on the surface. He
further holds that the periphery of the sphere, when seen in section,
has an appreciable thickness, its inner margin being definite and parallel
to the outer one ; and that the sides of the hexagons being continued
downwards through the thickness of the outer membraue, the appearance
of all these structures, if they could be seen simultaneously, would
be that shown in Plate 23, Fig. 6.
“ Even in deeply stained specimens I have never been able to detect
the existence of these hexagons as other than an entirely superjioial
structure, and at present my impression is that the hexagonal structure
has a different significance.
“ In the very early stage of Volvox-life the embryo gonidia are
encased in a distinct transparent oufcer-sphere. (Plate 23, Fig. 5,5a.) At
a later period, owing to the more rapid growth of the gonidia than of
the case, the latter closely invests the former, which are, in fact,
embedded in it. In the next stage, if not in the earlier condition, by
the continued growth of the gonidia at a greater rate than that of the
containing sphere, they are so closely appressed as to assume the hexagonal
form, and the interstices must of necessity consist of a thin film
of the substance of the containing sphærical envelope, moulded, so to
speak, into corresponding forms. But now the diameter of the young
Volvox, which is by this time sent forth on its independent career,
rapidly increases, the gonidia assuming their sphærical or pyriform
shape as their mutual pressure diminishes, and being hourly separated
by greater intervals. If, now, the actual formative m atter of the sphere
receives no further or only a disproportionate increment, but is
gradually attenuated by continued expansion, as a soap bubble is distended
by blowing into it, the hexagonal lines iuto which it has been
moulded by the previous mutual pressure of the embryo gonidia will be
gradually stretched in all directions into finer proportions ; and just as
this figure is that which is necessarily assumed by a number of sphærical
bodies under mutual pressure, so the most economical disposition of this
particular part of the Volvox-structure will necessitate its constant
attenuation into hexagons of ever-increasing delicacy. (Plate 22, Figs,
1 ,3 ; Plate 23, Fig. 7.) If the process be continued long enough, it
may finally result in the structure becoming too filmy to be detected by
any microscopical observation ; and it is worth noticing that it is
usually in spheres of small or medium diameter that the hexagonal
divisions can be developed, and not in those of the largest size. Such
appears to me at present to be the rationale of the formation of this
structure.
“ The internal cavity of the sphei’e is said to be filled with a ‘ mucilaginous
fluid.’ fif a Volvox be ruptured under a cover-glass, and aniline
purple introduced by capillary attraction, the colour seems to be for a
while repelled at that part which is in front of the rapture, and to flow
round it on either side. I t is only after a considerable time that it
gradually penetrates this space, and brings out, by staining it of a deep
purple tint, a mass of hazy matter, from w'hich proceed streaks or
lines radiating more or less regularly from its south pole. This structure,
to which I do not think attention has been hitherto called, is also sometimes
developed iu deeply stained specimens within the slightly
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