ON CONCRETIONS, SPICULES, AND SPICULAR SKELETONS
The deposition of inorganic material in the living body, usually in the
form of calcium salts or of silica, is a very common and wide-spread
phenomenon. It begins in simple ways, by the appearance of small
isolated particles, crystalline or non-crystalline, whose form has
little relation or sometimes none to the structure of the organism; it
culminates in the complex skeletons of the vertebrate animals, in the
massive skeletons of the corals, or in the polished, sculptured and
mathematically regular molluscan shells. Even among many very simple
organisms, such as the Diatoms, the Radiolarians, the Foraminifera,
or the Sponges, the skeleton displays extraordinary variety and
beauty, whether by reason of the intrinsic form of its elementary
constituents or the geometric symmetry with which these are arranged
and interconnected.
With regard to the form of these various structures (and this is
all that immediately concerns us here), it is plain that we have to
do with two distinct problems, which however, though theoretically
distinct, may merge with one another. For the form of the spicule or
other skeletal element may depend simply upon its chemical nature, as
for instance, to take a simple but not the only case, when the form is
purely crystalline; or the inorganic solid material may be laid down
in conformity with the shapes assumed by the cells, tissues or organs,
and so be, as it were, moulded to the shape of the living organism; and
again, there may well be intermediate stages in which both phenomena
may be simultaneously recognised, the molecular forces playing their
part in conjunction with, and under the restraint of, the other forces
inherent in the system. {412}
So far as the problem is a purely chemical one, we must deal with it
very briefly indeed; and all the more because special investigations
regarding it have as yet been few, and even the main facts of the case
are very imperfectly known. This at least is evident, that the whole
series of phenomena with which we are about to deal go deep into the
subject of colloid chemistry, and especially with that branch of the
science which deals with the properties of colloids in connection with
capillary or surface phenomena. It is to the special student of colloid
chemistry that we must ultimately and chiefly look for the elucidation
of our problem[406].
In the first and simplest part of our subject, the essential problem
is the problem of crystallisation in presence of colloids. In the
cells of plants, true crystals are found in comparative abundance,
and they consist, in the great majority of cases, of calcium oxalate.
In the stem and root of the rhubarb, for instance, in the leaf-stalk
of Begonia, and in countless other cases, sometimes within the cell,
sometimes in the substance of the cell-wall, we find large and
well-formed crystals of this salt; their varieties of form, which are
extremely numerous, are simply the crystalline forms proper to the salt
itself, and belong to the two systems, cubic and monoclinic, in one or
other of which, according to the amount of water of crystallisation,
this salt is known to crystallise. When calcium oxalate crystallises
according to the latter system (as it does when its molecule is
combined with two molecules of water of crystallisation), the
microscopic crystals have the form of fine needles, or “raphides,” such
as are very common in plants; and it has been found that these are
artificially produced when the salt is crystallised out in presence of
glucose or of dextrin[407].
[Illustration: Fig. 194. Alcyonarian spicules: _Siphonogorgia_ and
_Anthogorgia_. (After Studer.)]
Calcium carbonate, on the other hand, when it occurs in plant-cells (as
it does abundantly, for instance in the “cystoliths” of the Urticaceae
and Acanthaceae, and in great quantities in Melobesia {413} and the
other calcareous or “stony” algae), appears in the form of fine rounded
granules, whose inherent crystalline structure is not outwardly
visible, but is only revealed (like that of a molluscan shell) under
polarised light. Among animals, a skeleton of carbonate of lime occurs
under a multitude of forms, of which we need only mention now a very
few of the most conspicuous. The spicules of the calcareous sponges
are triradiate, occasionally quadriradiate, bodies, with pointed rays,
not crystalline in outward form but with a definitely crystalline
internal structure. We shall return again to these, and find for them
what would seem to be a satisfactory explanation of their form. Among
the Alcyonarian zoophytes we have a great variety of spicules[408],
which are sometimes straight and slender rods, sometimes flattened and
more or less striated plates, and still more often rounded or branched
concretions with rough or knobby surfaces (Figs. 194, 200). A third
type, presented by several very different things, such as a pearl, or
the ear-bone of a bony fish, consists of a more or less {414} rounded
body, sometimes spherical, sometimes flattened, in which the calcareous
matter is laid down in concentric zones, denser and clearer layers
alternating with one another. In the development of the molluscan shell
and in the calcification of a bird’s egg or the shell of a crab, for
instance, spheroidal bodies with similar concentric striation make
their appearance; but instead of remaining separate they become crowded
together, and as they coalesce they combine to form a pattern of
hexagons. In some cases, the carbonate of lime on being dissolved away
by acid leaves behind it a certain small amount of organic residue; in
most cases other salts, such as phosphates of lime, ammonia or magnesia
are present in small quantities; and in most cases if not all the
developing spicule or concretion is somehow or other so associated with
living cells that we are apt to take it for granted that it owes its
peculiarities of form to the constructive or plastic agency of these.
The appearance of direct association with living cells, however, is
apt to be fallacious; for the actual _precipitation_ takes place,
as a rule, not in actively living, but in dead or at least inactive
tissue[409]: that is to say in the “formed material” or matrix which
(as for instance in cartilage) accumulates round the living cells, in
the interspaces between these latter, or at least, as often happens,
in connection with the cell-wall or cell-membrane rather than within
the substance of the protoplasm itself. We need not go the length of
asserting that this is a rule without exception; but, so far as it
goes, it is of great importance and to its consideration we shall
presently return[410].
Cognate with this is the fact that it is known, at least in some
cases, that the organism can go on living and multiplying with
apparently unimpaired health, when stinted or even wholly deprived
of the material of which it is wont to make its spicules {415} or
its shell. Thus, Pouchet and Chabry[411] have shown that the eggs of
sea-urchins reared in lime-free water develop in apparent health, into
larvae entirely destitute of the usual skeleton of calcareous rods,
and in which, accordingly, the long arms of the Pluteus larva, which
the rods support and distend, are entirely suppressed. And again,
when Foraminifera are kept for generations in water from which they
gradually exhaust the lime, their shells grow hyaline and transparent,
and seem to consist only of chitinous material. On the other hand,
in the presence of excess of lime, the shells become much altered,
strengthened with various “ornaments,” and assuming characters
described as proper to other varieties and even species[412].
The crucial experiment, then, is to attempt the formation of similar
structures or forms, apart from the living organism: but, however
feasible the attempt may be in theory, we shall be prepared from the
first to encounter difficulties, and to realise that, though the
actions involved may be wholly within the range of chemistry and
physics, yet the actual conditions of the case may be so complex,
subtle and delicate, that only now and then, and in the simplest of
cases, shall we find ourselves in a position to imitate them completely
and successfully. Such an investigation is only part of that much
wider field of enquiry through which Stephane Leduc and many other
workers[413] have sought to produce, by synthetic means, forms similar
to those of living things; but it is a well-defined and circumscribed
part of that wider investigation. When by chemical or physical
experiment we obtain configurations similar, for instance, to the
phenomena of nuclear division, or conformations similar to a pattern of
hexagonal cells, or a group of vesicles which resemble some particular
tissue or cell-aggregate, we indeed prove what it is the main object of
this book to illustrate, namely, that the physical forces are capable
of producing particular organic forms. But it is by no means always
that we can feel perfectly assured that the physical forces which
we deal with in our experiment are identical with, and not merely
analogous to, {416} the physical forces which, at work in nature, are
bringing about the result which we have succeeded in imitating. In
the present case, however, our enquiry is restricted and apparently
simplified; we are seeking in the first instance to obtain by purely
chemical means a purely chemical result, and there is little room for
ambiguity in our interpretation of the experiment.
――――――――――
When we find ourselves investigating the forms assumed by chemical
compounds under the peculiar circumstances of association with a
living body, and when we find these forms to be characteristic or
recognisable, and somehow different from those which, under other
circumstances, the same substance is wont to assume, an analogy
presents itself to our minds, captivating though perhaps somewhat
remote, between this subject of ours and certain synthetic problems of
the organic chemist. There is doubtless an essential difference, as
well as a difference of scale, between the visible form of a spicule
or concretion and the hypothetical form of an individual molecule;
but molecular form is a very important concept; and the chemist has
not only succeeded, since the days of Wöhler, in synthesising many
substances which are characteristically associated with living matter,
but his task has included the attempt to account for the molecular
_forms_ of certain “asymmetric” substances, glucose, malic acid and
many more, as they occur in nature. These are bodies which, when
artificially synthesised, have no optical activity, but which, as we
actually find them in organisms, turn (when _in solution_) the plane
of polarised light in one direction or the other; thus dextro-glucose
and laevomalic acid are common products of plant metabolism; but
dextromalic acid and laevo-glucose do not occur in nature at all. The
optical activity of these bodies depends, as Pasteur shewed more than
fifty years ago[414], upon the form, right-handed or left-handed,
of their molecules, which molecular asymmetry further gives rise to
a corresponding right or left-handedness (or enantiomorphism) in
the crystalline aggregates. It is a distinct problem in organic or
physiological chemistry, {417} and by no means without its interest
for the morphologist, to discover how it is that nature, for each
particular substance, habitually builds up, or at least selects, its
molecules in a one-sided fashion, right-handed or left-handed as the
case may be. It will serve us no better to assert that this phenomenon
has its origin in “fortuity,” than to repeat the Abbé Galiani’s saying,
“_les dés de la nature sont pipés._”
The problem is not so closely related to our immediate subject that
we need discuss it at length; but at the same time it has its clear
relation to the general question of _form_ in relation to vital
phenomena, and moreover it has acquired interest as a theme of
long-continued discussion and new importance from some comparatively
recent discoveries.
According to Pasteur, there lay in the molecular asymmetry of the
natural bodies and the symmetry of the artificial products, one of the
most deep-seated differences between vital and non-vital phenomena:
he went further, and declared that “this was perhaps the _only_
well-marked line of demarcation that can at present [1860] be drawn
between the chemistry of dead and of living matter.” Nearly forty
years afterwards the same theme was pursued and elaborated by Japp in
a celebrated lecture[415], and the distinction still has its weight, I
believe, in the minds of many if not most chemists.
“We arrive at the conclusion,” said Professor Japp, “that the
production of single asymmetric compounds, or their isolation from
the mixture of their enantiomorphs, is, as Pasteur firmly held,
the prerogative of life. Only the living organism, or the living
intelligence with its conception of asymmetry, can produce this
result. Only asymmetry can beget asymmetry.” In these last words
(which, so far as the chemist and the biologist are concerned, we
may acknowledge to be perfectly true[416]) lies the {418} crux of
the difficulty; for they at once bid us enquire whether in nature,
external to and antecedent to life, there be not some asymmetry to
which we may refer the further propagation or “begetting” of the new
asymmetries: or whether in default thereof, we be rigorously confined
to the conclusion, from which Japp “saw no escape,” that “at the moment
when life first arose, a directive force came into play,—a force of
precisely the same character as that which enables the intelligent
operator, by the exercise of his will, to select one crystallised
enantiomorph and reject its asymmetric opposite[417].”
Observe that it is only the first beginnings of chemical asymmetry
that we need to discover; for when asymmetry is once manifested, it is
not disputed that it will continue “to beget asymmetry.” A plausible
suggestion is now at hand, which if it be confirmed and extended will
supply or at least sufficiently illustrate the kind of explanation
which is required[418].
We know in the first place that in cases where ordinary non-polarised
light acts upon a chemical substance, the amount of chemical action
is proportionate to the amount of light absorbed. We know in the
second place[419], in certain cases, that light circularly polarised
is absorbed in different amounts by the right-handed or left-handed
varieties, as the case may be, of an asymmetric substance. And thirdly,
we know that a portion of the light which comes to us from the sun
is already plane-polarised light, which becomes in part circularly
polarised, by reflection (according to Jamin) at the surface of the
sea, and then rotated in a particular direction under the influence of
terrestrial magnetism. We only require to be assured that the relation
between absorption of light and chemical activity will continue to hold
good in the case of circularly polarised light; that is to say {419}
that the formation of some new substance or other, under the influence
of light so polarised, will proceed asymmetrically in consonance
with the asymmetry of the light itself; or conversely, that the
asymmetrically polarised light will tend to more rapid decomposition
of those molecules by which it is chiefly absorbed. This latter proof
is now said to be furnished by Byk[420], who asserts that certain
tartrates become unsymmetrical under the continued influence of the
asymmetric rays. Here then we seem to have an example, of a particular
kind and in a particular instance, an example limited but yet crucial
(_if confirmed_), of an asymmetric force, non-vital in its origin,
which might conceivably be the starting-point of that asymmetry which
is characteristic of so many organic products.
The mysteries of organic chemistry are great, and the differences
between its processes or reactions as they are carried out in the
organism and in the laboratory are many[421]. The actions, catalytic
and other, which go on in the living cell are of extraordinary
complexity. But the contention that they are different in kind from
what we term ordinary chemical operations, or that in the production
of single asymmetric compounds there is actually to be witnessed, as
Pasteur maintained, a “prerogative of life,” would seem to be no longer
safely tenable. And furthermore, it behoves us to remember that, even
though failure continued to attend all artificial attempts to originate
the asymmetric or optically active compounds which organic nature
produces in abundance, this would only prove that a certain _physical
force_, or mode of _physical action_, is at work among living things
though unknown elsewhere. It is a mode of action which we can easily
imagine, though the actual mechanism we cannot set agoing when we
please. And it follows that such a difference between living matter and
dead would carry us but a little way, for it would still be confined
strictly to the physical or mechanical plane.
Our historic interest in the whole question is increased by the
{420} fact, or the great probability, that “the tenacity with which
Pasteur fought against the doctrine of spontaneous generation was
not unconnected with his belief that chemical compounds of one-sided
symmetry could not arise save under the influence of life[422].” But
the question whether spontaneous generation be a fact or not does
not depend upon theoretical considerations; our negative response is
based, and is so far soundly based, on repeated failures to demonstrate
its occurrence. Many a great law of physical science, not excepting
gravitation itself, has no higher claim on our acceptance.
――――――――――
Let us return then, after this digression, to the general subject
of the forms assumed by certain chemical bodies when deposited or
precipitated within the organism, and to the question of how far these
forms may be artificially imitated or theoretically explained.
Mr George Rainey, of St Bartholomew’s Hospital (to whom we have already
referred), and Professor P. Harting, of Utrecht, were the first to
deal with this specific problem. Mr Rainey published, between 1857 and
1861, a series of valuable and thoughtful papers to shew that shell and
bone and certain other organic structures were formed “by a process
of molecular coalescence, demonstrable in certain artificially-formed
products[423].” Professor Harting, after thirty years of experimental
work, published in 1872 a paper, which has become classical, entitled
_Recherches de Morphologie Synthétique, sur la production artificielle
de quelques formations calcaires organiques_; his aim was to pave the
way for a “morphologie synthétique,” as Wöhler had laid the foundations
of a “chimie synthétique,” by his classical discovery forty years
before. {421}
[Illustration: Fig. 195. Calcospherites, or concretions of calcium
carbonate, deposited in white of egg. (After Harting.)]
[Illustration: Fig. 196. A single calcospherite, with central
“nucleus,” and striated, iridescent border. (After Harting.)]
[Illustration: Fig. 197. Later stages in the same experiment.]
[Illustration: Fig. 198, A. Section of shell of Mya; B. Section of
hinge-tooth of do. (After Carpenter.)]
Rainey and Harting used similar methods, and these were such as many
other workers have continued to employ,—partly with the direct object
of explaining the genesis of organic forms and partly as an integral
part of what is now known as Colloid Chemistry. The whole gist of the
method was to bring some soluble salt of lime, such as the chloride
or nitrate, into solution within a colloid medium, such as gum,
gelatine or albumin; and then to precipitate it out in the form of
some insoluble compound, such as the carbonate or oxalate. Harting
found that, when he added a little sodium or potassium carbonate to a
concentrated solution of calcium chloride in albumin, he got at first
a gelatinous mass, or “colloid precipitate”: which slowly transformed
by the appearance of tiny microscopic particles, at first motionless,
but afterwards as they grew larger shewing the typical Brownian
movement. So far, very much the same phenomena were witnessed whether
the solution were albuminous or not, and similar appearances indeed had
been witnessed and recorded by Gustav Rose, so far back as 1837[424];
but in the later stages the presence of albuminoid matter made a great
difference. Now, after a few days, the calcium carbonate was seen to
be deposited in the form of large rounded concretions, with a more or
less distinct central nucleus, and with a surrounding structure at once
radiate and {422} concentric; the presence of concentric zones or
lamellae, alternately dark and clear, was especially characteristic.
These round “calcospherites” shewed a tendency to aggregate together in
layers, and then to assume polyhedral, or often regularly hexagonal,
outlines. In this latter condition they closely resemble the early
stages of calcification in a molluscan (Fig. 198), or still more in a
crustacean shell[425]; while in their isolated condition {423} they
very closely resemble the little calcareous bodies in the tissues of
a trematode or a cestode worm, or in the oesophageal glands of an
earthworm[426].
[Illustration: Fig. 199. Large irregular calcareous concretions, or
spicules, deposited in a piece of dead cartilage, in presence of
calcium phosphate. (After Harting.)]
When the albumin was somewhat scanty, or when it was mixed with
gelatine, and especially when a little phosphate of lime was {424}
added to the mixture, the spheroidal globules tended to become rough,
by an outgrowth of spinous or digitiform projections; and in some
cases, but not without the presence of the phosphate, the result was an
irregularly shaped knobby spicule, precisely similar to those which are
characteristic of the Alcyonaria[427].
[Illustration: Fig. 200. Additional illustrations of Alcyonarian
spicules: _Eunicea_. (After Studer.)]
The rough spicules of the Alcyonaria are extraordinarily variable
in shape and size, as, looking at them from the chemist’s or the
physicist’s point of view, we should expect them to be. Partly upon
the form of these spicules, and partly on the general form or mode of
branching of the entire colony of polypes, a vast number of separate
“species” have been based by systematic zoologists. But it is now
admitted that even in specimens of a single species, from one and the
same locality, the spicules may vary immensely in shape and size: and
Professor Hickson declares (in a paper published while these sheets
are passing through the press) that after many years of laborious work
in striving to determine species of these animal colonies, he feels
“quite convinced that we have been engaged in a more or less fruitless
task[428]”.
The formation of a tooth has very lately been shown to be a phenomenon
of the same order. That is to say, “calcification in both dentine
and enamel {425} is in great part a physical phenomenon; the actual
deposit in both tissues occurs in the form of calcospherites, and the
process in mammalian tissue is identical in every point with the same
process occurring in lower organisms[429].” The ossification of bone,
we may be sure, is in the same sense and to the same extent a physical
phenomenon.
The typical structure of a calcospherite is no other than that of
a pearl, nor does it differ essentially from that of the otolith
of a mollusc or of a bony fish. (The otoliths, by the way, of the
elasmobranch fishes, like those of reptiles and birds, are not
developed after this fashion, but are true crystals of calc-spar.)
[Illustration: Fig. 201. A “crust” of close-packed calcareous
concretions, precipitated at the surface of an albuminous solution.
(After Harting.)]
Throughout these phenomena, the effect of surface-tension is manifest.
It is by surface-tension that ultra-microscopic particles are brought
together in the first floccular precipitate or coagulum; by the same
agency, the coarser particles are in turn agglutinated into visible
lumps; and the form of the calcospherites, whether it be that of the
solitary spheres or that assumed in various stages of aggregation (e.g.
Fig. 202)[430], is likewise due to the same agency.
[Illustration: Fig. 202. Aggregated calcospherites. (After Harting.)]
From the point of view of colloid chemistry the whole phenomenon is
very important and significant; and not the least significant part
is this tendency of the solidified deposits to assume the form of
“spherulites,” and other rounded contours. In the phraseology of that
science, we are dealing with a _two-phase_ system, which finally
consists of solid particles in suspension in a liquid (the former
being styled the _disperse phase_, the latter the {426} _dispersion
medium_). In accordance with a rule first recognised by Ostwald[431],
when a substance begins to separate out from a solution, so making
its appearance as a _new phase_, it always makes its appearance first
as a liquid[432]. Here is a case in point. The minute quantities
of material, on their way from a state of solution to a state of
“suspension,” pass through a liquid to a solid form; and their
temporary sojourn in the former leaves its impress in the rounded
contours which surface-tension brought about while the little aggregate
was still labile or fluid: while coincidently with this surface-tension
effect upon the surface, crystallisation tended to take place
throughout the little liquid mass, or in such portion of it as had not
yet consolidated and crystallised.
[Illustration: Fig. 203. (After Harting.)]
Where we have simple aggregates of two or three calcospherites, the
resulting figure is precisely that of so many contiguous soap-bubbles.
In other cases, composite forms result which are not so easily
explained, but which, if we could only account for them, would be
of very great interest to the biologist. For instance, when smaller
calcospheres seem, as it were, to invade the substance of a larger one,
we get curious conformations which in the closest possible way resemble
the outlines of certain of the Diatoms (Fig. 203). Another very
curious formation, which Harting calls a “conostat,” is of frequent
occurrence, and in it we see at least a suggestion of analogy with
the configuration which, in a protoplasmic structure, we have spoken
of as a “collar-cell.” The {427} conostats, which are formed in the
surface layer of the solution, consist of a portion of a spheroidal
calcospherite, whose upper part is continued into a thin spheroidal
collar, of somewhat larger radius than the solid sphere; but the
precise manner in which the collar is formed, possibly around a bubble
of gas, possibly about a vortex-like diffusion-current[433] is not
obvious.
――――――――――
Among these various phenomena, the concentric striation observed in
the calcospherite has acquired a special interest and importance[434].
It is part of a phenomenon now widely known, and recognised as an
important factor in colloid chemistry, under the name of “Liesegang’s
Rings[435].”
[Illustration: Fig. 204. Conostats. (After Harting.)]
If we dissolve, for instance, a little bichromate of potash in
gelatine, pour it on to a glass plate, and after it is set place upon
it a drop of silver nitrate solution, there appears in the course of
a few hours the phenomenon of Liesegang’s rings. At first the silver
forms a central patch of abundant reddish brown chromate precipitate;
but around this, as the silver nitrate diffuses slowly through the
gelatine, the precipitate no longer comes down in a continuous,
uniform layer, but forms a series of zones, beautifully regular, which
alternate with clear interspaces of jelly, and which stand farther and
farther apart, in logarithmic ratio, as they recede from the centre.
For a discussion of the _raison d’être_ of {428} this phenomenon,
still somewhat problematic, the student must consult the text-books of
physical and colloid chemistry[436].
But, speaking very generally, we may say the appearance of Liesegang’s
rings is but a particular and striking case of a more general
phenomenon, namely the influence on crystallisation of the presence of
foreign bodies or “impurities,” represented in this case by the “gel”
or colloid matrix[437]. Faraday shewed long ago that to the presence
of slight impurities might be ascribed the banded structure of ice,
of banded quartz or agate, onyx, etc.; and Quincke and Tomlinson have
added to our scanty knowledge of the same phenomenon[438].
[Illustration: Fig. 205. Liesegang’s Rings. (After Leduc.)]
Besides the tendency to rhythmic action, as manifested in Liesegang’s
rings, the association of colloid matter with a crystalloid in solution
may lead to other well-marked effects. These, according to Professor
J. H. Bowman[439], may be grouped somewhat as follows: (1) total
prevention of crystallisation; (2) suppression of certain of the
lines of crystalline growth; (3) extension of the crystal to abnormal
proportions, with a tendency for it to become a compound crystal; (4) a
curving or gyrating of the crystal or its parts. {429}
[Illustration: Fig. 206. Relay-crystals of common salt. (After
Bowman.)]
[Illustration: Fig. 207. Wheel-like crystals in a colloid. (After
Bowman.)]
For instance, it would seem that, if the supply of material to the
growing crystal be not forthcoming in sufficient quantity (as may well
happen in a colloid medium, for lack of convection-currents), then
growth will follow only the strongest lines of crystallising force,
and will be suppressed or partially suppressed along other axes.
The crystal will have a tendency to become filiform, or “fibrous”;
and the raphides of our plant-cells are a case in point. Again, the
long slender crystal so formed, pushing its way into new material,
may initiate a new centre of crystallisation: we get the phenomenon
known as a “relay,” along the principal lines of force, and sometimes
along subordinate axes as well. This phenomenon is illustrated in the
accompanying figure of crystallisation in a colloid medium of common
salt; and it may possibly be that we have here an explanation, or
part of an explanation, of the compound siliceous spicules of the
Hexactinellid sponges. Lastly, when the crystallising force is nearly
equalled by the resistance of the viscous medium, the crystal takes
the line of least resistance, with very various results. One of these
results would seem to be a gyratory course, giving to the crystal a
curious wheel-like shape, as in Fig. 207; and other results are the
feathery, fern-like {430} or arborescent shapes so frequently seen in
microscopic crystallisation.
To return to Liesegang’s rings, the typical appearance of concentric
rings upon a gelatinous plate may be modified in various experimental
ways. For instance, our gelatinous medium may be placed in a capillary
tube immersed in a solution of the precipitating salt, and in this
case we shall obtain a vertical succession of bands or zones regularly
interspaced: the result being very closely comparable to the banded
pigmentation which we see in the hair of a rabbit or a rat. In the
ordinary plate preparation, the free surface of the gelatine is under
different conditions to the lower layers and especially to the lowest
layer in contact with the glass; and therefore it often happens that we
obtain a double series of rings, one deep and the other superficial,
which by occasional blending or interlacing, may produce a netted
pattern. In some cases, as when only the inner surface of our capillary
tube is covered with a layer of gelatine, there is a tendency for the
deposit to take place in a continuous spiral line, rather than in
concentric and separate zones. By such means, according to Küster[440]
various forms of annular, spiral and reticulated thickenings in
the vascular tissue of plants may be closely imitated; and he and
certain other writers have of late been inclined to carry the same
chemico-physical phenomenon a very long way, in the explanation of
various banded, striped, and other rhythmically successional types of
structure or pigmentation. For example, the striped pigmentation of
the leaves in many plants (such as _Eulalia japonica_), the striped or
clouded colouring of many feathers or of a cat’s skin, the patterns
of many fishes, such for instance as the brightly coloured tropical
Chaetodonts and the like, are all regarded by him as so many instances
of “diffusion-figures” closely related to the typical Liesegang
phenomenon. Gebhardt has made a particular study of the same subject in
the case of insects[441]. He declares, for instance, that the banded
wings of _Papilio podalirius_ are precisely imitated in Liesegang’s
experiments; that the finer markings on the wings of the Goatmoth
(_Cossus ligniperda_) shew the double arrangement of larger and of
{431} smaller intermediate rhythms, likewise manifested in certain
cases of the same kind; that the alternate banding of the antennae (for
instance in _Sesia spheciformis_), a pigmentation not concurrent with
the segmented structure of the antenna, is explicable in the same way;
and that the “ocelli,” for instance of the Emperor moth, are typical
illustrations of the common concentric type. Darwin’s well-known
disquisition[442] on the ocellar pattern of the feathers of the Argus
Pheasant, as a result of sexual selection, will occur to the reader’s
mind, in striking contrast to this or to any other direct physical
explanation[443]. To turn from the distribution of pigment to more
deeply seated structural characters, Leduc has shewn how, for instance,
the laminar structure of the cornea or the lens is again, apparently,
a similar phenomenon. In the lens of the fish’s eye, we have a very
curious appearance, the consecutive lamellae being roughened or
notched by close-set, interlocking sinuosities; and precisely the same
appearance, save that it is not quite so regular, is presented in one
of Küster’s figures as the effect of precipitating a little sodium
phosphate in a gelatinous medium. Biedermann has studied, from the
same point of view, the structure and development of the molluscan
shell, the problem which Rainey had first attacked more than fifty
years before[444]; and Liesegang himself has applied his results to the
formation of pearls, and to the development of bone[445]. {432}
Among all the many cases where this phenomenon of Liesegang’s
comes to the naturalist’s aid in explanation of rhythmic or zonary
configurations in organic forms, it has a special interest where the
presence of concentric zones or rings appears, at first sight, as
a sure and certain sign of periodicity of growth, depending on the
seasons, and capable therefore of serving as a mark and record of the
creature’s age. This is the case, for instance, with the scales, bones
and otoliths of fishes; and a kindred phenomena in starch-grains has
given rise, in like manner, to the belief that they indicate a diurnal
and nocturnal periodicity of activity and rest[446].
[Illustration: Fig. 208.]
That this is actually the case in growing starch-grains is generally
believed, on the authority of Meyer[447]; but while under certain
circumstances a marked alternation of growing and resting periods
may occur, and may leave its impress on the structure of the grain,
there is now great reason to believe that, apart from such external
influences, the internal phenomena of diffusion may, just as in the
typical Liesegang experiment, produce the well-known concentric
rings. The spherocrystals of inulin, in like manner, shew, like the
“calcospherites” of Harting (Fig. 208), a concentric structure which in
all likelihood has had no causative impulse save from within.
[Illustration: Fig. 209. Otoliths of Plaice, showing four zones or
“age-rings.” (After Wallace.)]
The striation, or concentric lamellation, of the scales and otoliths
of fishes has been much employed of recent years as a trustworthy and
unmistakeable mark of the fish’s age. There are difficulties in the
way of accepting this hypothesis, not the least of which is the fact
that the otolith-zones, for instance, are extremely well marked even in
the case of some fishes which spend their lives in deep water, {433}
where the temperature and other physical conditions shew little or
no appreciable fluctuation with the seasons of the year. There are,
on the other hand, phenomena which seem strongly confirmatory of the
hypothesis: for instance the fact (if it be fully established) that
in such a fish as the cod, zones of growth, _identical in number_,
are found both on the scales and in the otoliths[448]. The subject
has become a much debated one, and this is not the place for its
discussion; but it is at least obvious, with the Liesegang phenomenon
in view, that we have no right to _assume_ that an appearance of rhythm
and periodicity in structure and growth is necessarily bound up with,
and indubitably brought about by, a periodic recurrence of particular
_external_ conditions.
But while in the Liesegang phenomenon we have rhythmic precipitation
which depends only on forces intrinsic to the system, and is
independent of any corresponding rhythmic changes in temperature or
other external conditions, we have not far to seek for instances of
chemico-physical phenomena where rhythmic alternations of appearance
or structure are produced in close relation to periodic fluctuations
of temperature. A well-known instance is that of the Stassfurt
deposits, where the rock-salt alternates regularly with thin layers of
“anhydrite,” or (in another series of beds) with “polyhalite[449]”: and
where these zones are commonly regarded as marking years, and their
alternate bands as having been formed in connection with the seasons.
A discussion, however, of this remarkable and significant phenomenon,
and of how the chemist explains it, by help of the “phase-rule,” in
connection with temperature conditions, would lead us far beyond our
scope[450].
――――――――――
We now see that the methods by which we attempt to study the chemical
or chemico-physical phenomena which accompany the development of an
inorganic concretion or spicule within the {434} body of an organism
soon introduce us to a multitude of kindred phenomena, of which our
knowledge is still scanty, and which we must not attempt to discuss
at greater length. As regards our main point, namely the formation
of spicules and other elementary skeletal forms, we have seen that
certain of them may be safely ascribed to simple precipitation or
crystallisation of inorganic materials, in ways more or less modified
by the presence of albuminous or other colloid substances. The effect
of these latter is found to be much greater in the case of some
crystallisable bodies than in others. For instance, Harting, and Rainey
also, found as a rule that calcium oxalate was much less affected by a
colloid medium than was calcium carbonate; it shewed in their hands no
tendency to form rounded concretions or “calcospherites” in presence
of a colloid, but continued to crystallise, either normally, or with a
tendency to form needles or raphides. It is doubtless for this reason
that, as we have seen, _crystals_ of calcium oxalate are so common in
the tissues of plants, while those of other calcium salts are rare. But
true calcospherites, or spherocrystals, of the oxalate are occasionally
found, for instance in certain Cacti, and Bütschli[451] has succeeded
in making them artificially in Harting’s usual way, that is to say by
crystallisation in a colloid medium.
There link on to these latter observations, and to the statement
already quoted that calcareous deposits are associated with the dead
products rather than with the living cells of the organism, certain
very interesting facts in regard to the _solubility_ of salts in
colloid media, which have been made known to us of late, and which go
far to account for the presence (apart from the form) of calcareous
precipitates within the organism[452]. It has been shewn, in the
first place, that the presence of albumin has a notable effect on
the solubility in a watery solution of calcium salts, increasing
the solubility of the phosphate in a marked degree, and that of the
carbonate in still greater proportion; but the {435} sulphate is only
very little more soluble in presence of albumin than in pure water, and
the rarity of its occurrence within the organism is so far accounted
for. On the other hand, the bodies derived from the breaking down of
the albumins, their “catabolic” products, such as the peptones, etc.,
dissolve the calcium salts to a much less degree than albumin itself;
and in the case of the phosphate, its solubility in them is scarcely
greater than in water. The probability is, therefore, that the actual
precipitation of the calcium salts is not due to the direct action
of carbonic acid, etc. on a more soluble salt (as was at one time
believed); but to catabolic changes in the proteids of the organism,
which tend to throw down the salts already formed, which had remained
hitherto in albuminous solution. The very slight solubility of calcium
phosphate under such circumstances accounts for its predominance in,
for instance, mammalian bone[453]; and wherever, in short, the supply
of this salt has been available to the organism.
To sum up, we see that, whether from food or from sea-water, calcium
sulphate will tend to pass but little into solution in the albuminoid
substances of the body: calcium carbonate will enter more freely, but a
considerable part of it will tend to remain in solution: while calcium
phosphate will pass into solution in considerable amount, but will be
almost wholly precipitated again, as the albumin becomes broken down in
the normal process of metabolism.
We have still to wait for a similar and equally illuminating study of
the solution and precipitation of _silica_, in presence of organic
colloids.
――――――――――
From the comparatively small group of inorganic formations which,
arising within living organisms, owe their form solely to precipitation
or to crystallisation, that is to say to chemical or other molecular
forces, we shall presently pass to that other and larger group which
appear to be conformed in direct relation to the forms and the
arrangement of the cells or other protoplasmic elements[454]. {436}
The two principles of conformation are both illustrated in the
spicular skeletons of the Sponges.
[Illustration: Fig. 210. Close-packed calcospherites, or so-called
“spicules,” of Astrosclera. (After Lister.)]
In a considerable number, but withal a minority of cases, the form
of the sponge-spicule may be deemed sufficiently explained on the
lines of Harting’s and Rainey’s experiments, that is to say as the
direct result of chemical or physical phenomena associated with the
deposition of lime or of silica in presence of colloids[455]. This is
the case, for instance, with various small spicules of a globular or
spheroidal form, formed of amorphous silica, concentrically striated
within, and often developing irregular knobs or tiny tubercles over
their surfaces. In the aberrant sponge _Astrosclera_[456], we have,
to begin with, rounded, striated discs or globules, which in like
manner are nothing more or less than the {437} “calcospherites” of
Harting’s experiments; and as these grow they become closely aggregated
together (Fig. 210), and assume an angular, polyhedral form, once more
in complete accordance with the results of experiment[457]. Again,
in many Monaxonid sponges, we have irregularly shaped, or branched
spicules, roughened or tuberculated by secondary superficial deposits,
and reminding one of the spicules of some Alcyonaria. These also must
be looked upon as the simple result of chemical deposition, the form of
the deposit being somewhat modified in conformity with the surrounding
tissues, just as in the simple experiment the form of the concretionary
precipitate is affected by the heterogeneity, visible or invisible,
of the matrix. Lastly, the simple needles of amorphous silica, which
constitute one of the commonest types of spicule, call for little
in the way of explanation; they are accretions or deposits about a
linear axis, or fine thread of organic material, just as the ordinary
rounded calcospherite is deposited about some minute point or centre of
crystallisation, and as ordinary crystallisation is often started by a
particle of atmospheric dust; in some cases they also, like the others,
are apt to be roughened by more irregular secondary deposits, which
probably, as in Harting’s experiments, appear in this irregular form
when the supply of material has become relatively scanty.
――――――――――
Our few foregoing examples, diverse as they are in look and kind and
ranging from the spicules of Astrosclera or Alcyonium to the otoliths
of a fish, seem all to have their free origin in some larger or smaller
fluid-containing space, or cavity of the body: pretty much as Harting’s
calcospheres made their appearance in the albuminous content of a dish.
But we now come at last to a much larger class of spicular and skeletal
structures, for whose regular and often complex forms some other
explanation than the intrinsic forces of crystallisation or molecular
adhesion is manifestly necessary. As we enter on this subject, which
is certainly no small or easy one, it may conduce to simplicity, and
to brevity, {438} if we try to make a rough classification, by way of
forecast, of the chief conditions which we are likely to meet with.
Just as we look upon animals as constituted, some of a vast number of
cells, and others of a single cell or of a very few, and just as the
shape of the former has no longer a visible relation to the individual
shapes of its constituent cells, while in the latter it is cell-form
which dominates or is actually equivalent to the form of the organism,
so shall we find it to be, with more or less exact analogy, in the
case of the skeleton. For example, our own skeleton consists of bones,
in the formation of each of which a vast number of minute living
cellular elements are necessarily concerned; but the form and even the
arrangement of these bone-forming cells or corpuscles are monotonously
simple, and we cannot find in these a physical explanation of the
outward and visible configuration of the bone. It is as part of a far
larger field of force,—in which we must consider gravity, the action of
various muscles, the compressions, tensions and bending moments due to
variously distributed loads, the whole interaction of a very complex
mechanical system,—that we must explain (if we are to explain at all)
the configuration of a bone.
In contrast to these massive skeletons, or constituents of a skeleton,
we have other skeletal elements whose whole magnitude, or whose
magnitude in some dimension or another, is commensurate with the
magnitude of a single living cell, or (as comes to very much the same
thing) is comparable to the range of action of the molecular forces.
Such is the case with the ordinary spicules of a sponge, with the
delicate skeleton of a Radiolarian, or with the denser and robuster
shells of the Foraminifera. The effect of _scale_, then, of which
we had so much to say in our introductory chapter on Magnitude, is
bound to be apparent in the study of skeletal fabrics, and to lead to
essential differences between the big and the little, the massive and
the minute, in regard to their controlling forces and their resultant
forms. And if all this be so, and if the range of action of the
molecular forces be in truth the important and fundamental thing, then
we may somewhat extend our statement of the case, and include in it not
only association with the living cellular elements of the body, but
also association with any bubbles, drops, vacuoles or vesicles which
{439} may be comprised within the bounds of the organism, and which
are (as their names and characters connote) of the order of magnitude
of which we are speaking.
Proceeding a little farther in our classification, we may conceive
each little skeletal element to be associated, in one case, with a
single cell or vesicle, and in another with a cluster or “system” of
consociated cells. In either case there are various possibilities.
For instance, the calcified or other skeletal material may tend to
overspread the entire outer surface of the cell or cluster of cells,
and so tend accordingly to assume some configuration comparable to
that of a fluid drop or of an aggregation of drops; this, in brief, is
the gist and essence of our story of the foraminiferal shell. Another
common, but very different condition will arise if, in the case of
the cell-aggregates, the skeletal material tends to accumulate in the
interstices _between_ the cells, in the partition-walls which separate
them, or in the still more restricted distribution indicated by the
_lines_ of junction between these partition-walls. Conditions such as
these will go a very long way to help us in our understanding of many
sponge-spicules and of an immense variety of radiolarian skeletons. And
lastly (for the present), there is a possible and very interesting case
of a skeletal element associated with the surface of a cell, not so as
to cover it like a shell, but only so as to pursue a course of its own
within it, and subject to the restraints imposed by such confinement to
a curved and limited surface. With this curious condition we shall deal
immediately.
This preliminary and much simplified classification of skeletal forms
(as is evident enough) does not pretend to completeness. It leaves out
of account some kinds of conformation and configuration with which
we shall attempt to deal, and others which we must perforce omit.
But nevertheless it may help to clear or to mark our way towards the
subjects which this chapter has to consider, and the conditions by
which they are at least partially defined.
――――――――――
Among the several possible, or conceivable, types of microscopic
skeletons let us choose, to begin with, the case of a spicule, more
or less simply linear as far as its _intrinsic_ powers of growth are
{440} concerned, but which owes its now somewhat complicated form to
a restraint imposed by the individual cell to which it is confined,
and within whose bounds it is generated. The conception of a spicule
developed under such conditions we owe to a distinguished physicist,
the late Professor G. F. FitzGerald.
Many years ago, Sollas pointed out that if a spicule begin to grow in
some particular way, presumably under the control or constraint imposed
by the organism, it continues to grow by further chemical deposition in
the same form or direction even after it has got beyond the boundaries
of the organism or its cells. This phenomenon is what we see in, and
this imperfect explanation goes so far to account for, the continued
growth in straight lines of the long calcareous spines of Globigerina
or Hastigerina, or the similarly radiating but siliceous spicules of
many Radiolaria. In physical language, if our crystalline structure has
once begun to be laid down in a definite orientation, further additions
tend to accrue in a like regular fashion and in an identical direction;
and this corresponds to the phenomenon of so-called “orientirte
Adsorption,” as described by Lehmann.
In Globigerina or in Acanthocystis the long needles grow out freely
into the surrounding medium, with nothing to impede their rectilinear
growth and their approximately radiate distribution. But let us
consider some simple cases to illustrate the forms which a spicule will
tend to assume when, striving (as it were) to grow straight, it comes
under the influence of some simple and constant restraint or compulsion.
If we take any two points on some curved surface, such as that of a
sphere or an ellipsoid, and imagine a string stretched between them,
we obtain what is known in mathematics as a “geodetic” curve. It is
the shortest line which can be traced between the two points, upon the
surface itself; and the most familiar of all cases, from which the name
is derived, is that curve upon the earth’s surface which the navigator
learns to follow in the practice of “great-circle sailing.” Where
the surface is spherical, the geodetic is always literally a “great
circle,” a circle, that is to say, whose centre is the centre of the
sphere. If instead of a sphere we be dealing with an ellipsoid, the
geodetic becomes a variable figure, according to the position of our
two points. {441} For obviously, if they lie in a line perpendicular
to the long axis of the ellipsoid, the geodetic which connects them is
a circle, also perpendicular to that axis; and if they lie in a line
parallel to the axis, their geodetic is a portion of that ellipse about
which the whole figure is a solid of revolution. But if our two points
lie, relatively to one another, in any other direction, then their
geodetic is part of a spiral curve in space, winding over the surface
of the ellipsoid.
To say, as we have done, that the geodetic is the shortest line
between two points upon the surface, is as much as to say that it is
a _projection_ of some particular straight line upon the surface in
question; and it follows that, if any linear body be confined to that
surface, while retaining a tendency to grow by successive increments
always (save only for its confinement to that surface) in a straight
line, the resultant form which it will assume will be that of a
geodetic. In mathematical language, it is a property of a geodetic that
the plane of any two consecutive elements is a plane perpendicular
to that in which the geodetic lies; or, in simpler words, any two
consecutive elements lie in a straight line _in the plane of the
surface_, and only diverge from a straight line in space by the actual
curvature of the surface to which they are restrained.
[Illustration: Fig. 211. Sponge and Holothurian spicules.]
[Illustration: Fig. 212.]
[Illustration: Fig. 213. An “amphidisc” of Hyalonema.]
Let us now imagine a spicule, whose natural tendency is to grow into
a straight linear element, either by reason of its own molecular
anisotropy, or because it is deposited about a thread-like axis; and
let us suppose that it is confined either within a cell-wall or in
adhesion thereto; it at once follows that its line of growth will be
simply a geodetic to the surface of the cell. And if the cell be an
imperfect sphere, or a more or less regular ellipsoid, the spicule will
tend to grow into one or other of three forms: either a plane curve
of circular arc; or, more commonly, a plane curve which is a portion
of an ellipse; or, most commonly of all, a curve which is a portion
of a spiral in space. In the latter case, the number of turns of the
spiral will depend, not only on the length of the spicule, but on
the relative dimensions of the ellipsoidal cell, as well as upon the
angle by which the spicule is inclined to the ellipsoid axes; but a
very common case will probably be that in which the spicule looks at
first sight to be {442} a plane C-shaped figure, but is discovered,
on more careful inspection, to lie not in one plane but in a more
complicated spiral twist. This investigation includes a series of
forms which are abundantly represented among actual sponge-spicules,
as illustrated in Figs. 211 and 212. If the spicule be not restricted
to linear growth, but have a tendency to expand, or to branch out from
a main axis, we shall obtain a series of more complex figures, all
related to the geodetic system of curves. A very simple case will arise
where the spicule occupies, in the first instance, the axis of the
containing cell, and then, on reaching its boundary, tends to branch
or spread outwards. We shall now get various figures, in some of which
the spicule will appear as an axis expanding into a disc or wheel at
either end; and in other cases, the terminal disc will be replaced, or
represented, by a series of rays or spokes, with a reflex curvature,
corresponding to the spherical or ellipsoid curvature of the surface
of the cell. Such spicules as these are again exceedingly common among
various sponges (Fig. 213).
Furthermore, if these mechanical methods of conformation, and others
like to these, be the true cause of the shapes which the spicules
assume, it is plain that the production of these spicular shapes is not
a specific function of sponges or of any particular sponge, but that
we should expect {443} the same or very similar phenomena to occur
in other organisms, wherever the conditions of inorganic secretion
within closed cells was very much the same. As a matter of fact, in the
group of Holothuroidea, where the formation of intracellular spicules
is a characteristic feature of the group, all the principal types of
conformation which we have just described can be closely paralleled.
Indeed in many cases, the forms of the Holothurian spicules are
identical and indistinguishable from those of the sponges[458]. But
the Holothurian spicules are composed of calcium carbonate while those
which we have just described in the case of sponges are usually, if not
always, siliceous: this being just another proof of the fact that in
such cases the form of the spicule is not due to its chemical nature or
molecular structure, but to the external forces to which, during its
growth, the spicule is submitted.
――――――――――
So much for that comparatively limited class of sponge-spicules
whose forms seem capable of explanation on the hypothesis that
they are developed within, or under the restraint imposed by, the
surface of a cell or vesicle. Such spicules are usually of small
size, as well as of comparatively simple form; and they are greatly
outstripped in number, in size, and in supposed importance as guides
to zoological classification, by another class of spicules. This new
class includes such as we have supposed to be capable of explanation
on the assumption that they develop in association (of some sort
or another) with the _lines of junction_ of contiguous cells. They
include the triradiate spicules of the calcareous sponges, the
quadriradiate or “tetractinellid” spicules which occur in the same
group, but more characteristically in certain siliceous sponges known
as the Tetractinellidae, and lastly perhaps (though these last are
admittedly somewhat harder to understand) the six-rayed spicules of the
Hexactinellids.
The spicules of the calcareous sponges are commonly triradiate, and the
three radii are usually inclined to one another at equal, or nearly
equal angles; in certain cases, two of the three rays are nearly in
a straight line, and at right angles to the {444} third[459]. They
are seldom in a plane, but are usually inclined to one another in a
solid, trihedral angle, not easy of precise measurement under the
microscope. The three rays are very often supplemented by a fourth,
which is set tetrahedrally, making, that is to say, coequal angles with
the other three. The calcareous spicule consists mainly of carbonate
of lime, in the form of calcite, with (according to von Ebner) some
admixture of soda and magnesia, of sulphates and of water. According
to the same writer (but the fact, though it would seem easy to test,
is still disputed) there is no organic matter in the spicule, either
in the form of an axial filament or otherwise, and the appearance
of stratification, often simulating the presence of an axial fibre,
is due to “mixed crystallisation” of the various constituents. The
spicule is a true crystal, and therefore its existence and its form are
_primarily_ due to the molecular forces of crystallisation; moreover
it is a single crystal and not a group of crystals, as is at once seen
by its behaviour in polarised light. But its axes are not crystalline
axes, and its form neither agrees with, nor in any way resembles,
any one of the many polymorphic forms in which calcite is capable of
crystallising. It is as though it were carved out of a solid crystal;
it is, in fact, a crystal under restraint, a crystal growing, as it
were, in an artificial mould; and this mould is constituted by the
surrounding cells, or structural vesicles of the sponge.
[Illustration: Fig. 214. Spicules of Grantia and other calcareous
sponges. (After Haeckel.)]
We have already studied in an elementary way, but amply for our
present purpose, the manner in which three or more cells, or bubbles,
tend to meet together under the influence of surface-tension, and
also the outwardly similar phenomena which may be brought about by a
uniform distribution of mechanical pressure. We have seen that when we
confine ourselves to a plane assemblage of such bodies, we find them
meeting one another in threes; that in a section or plane projection
of such an assemblage we see the partition-walls meeting one another
at equal angles of 120°; that when the bodies are uniform in size, the
partitions are straight lines, which combine to form regular hexagons;
and that when {445} the bodies are unequal in size, the partitions
are curved, and combine to form other and less regular polygons. It
is plain, accordingly, that in any flattened or stratified assemblage
of such cells, a solidified skeletal deposit which originates or
accumulates either between the cells or within the thickness of their
mutual partitions, will tend to take the form of triradiate bodies,
whose rays (in a typical case) will be set at equal angles of 120°
(Fig. 214, _F_). And this latter condition of equality will be open to
modification in various ways. It will be modified by any inequality
in the specific tensions of adjacent cells; as a special case, it
will be apt to be greatly modified at the surface of the system,
where a spicule happens to be formed in a plane perpendicular to the
cell-layer, so that one of its three rays lies between two adjacent
cells and the other two are associated with the surface of contact
between the cells and the surrounding medium; in such a case (as in the
cases considered in connection with the forms of the cells themselves
{446} on p. 314), we shall tend to obtain a spicule with two equal
angles and one unequal (Fig. 214, _A_, _C_). In the last case, the two
outer, or superficial rays, will tend to be markedly curved. Again, the
equiangular condition will be departed from, and more or less curvature
will be imparted to the rays, wherever the cells of the system cease
to be uniform in size, and when the hexagonal symmetry of the system
is lost accordingly. Lastly, although we speak of the rays as meeting
at certain definite angles, this statement applies to their _axes_,
rather than to the rays themselves. For, if the triradiate spicule be
developed in the _interspace_ between three juxtaposed cells, it is
obvious that its sides will tend to be concave, for the interspace
between our three contiguous equal circles is an equilateral,
curvilinear triangle; and even if our spicule be deposited, not in the
space between our three cells, but in the thickness of the intervening
wall, then we may recollect (from p. 297) that the several partitions
never actually meet at sharp angles, but the angle of contact is always
bridged over by a small accumulation of material (varying in amount
according to its fluidity) whose boundary takes the form of a circular
arc, and which constitutes the “bourrelet” of Plateau.
In any sample of the triradiate spicules of Grantia, or in any series
of careful drawings, such as those of Haeckel among others, we shall
find that all these various configurations are precisely and completely
illustrated.
The tetrahedral, or rather tetractinellid, spicule needs no explanation
in detail (Fig. 214, _D_, _E_). For just as a triradiate spicule
corresponds to the case of three cells in mutual contact, so does the
four-rayed spicule to that of a solid aggregate of four cells: these
latter tending to meet one another in a tetrahedral system, shewing
four edges, at each of which four surfaces meet, the edges being
inclined to one another at equal angles of about 109°. And even in the
case of a single layer, or superficial layer, of cells, if the skeleton
originate in connection with all the edges of mutual contact, we shall,
in complete and typical cases, have a four-rayed spicule, of which one
straight limb will correspond to the line of junction between the three
cells, and the other three limbs (which will then be curved limbs) will
correspond to the edges where two cells meet one another on the surface
of the system. {447}
But if such a physical explanation of the forms of our spicules is
to be accepted, we must seek at once for some physical agency by
which we may explain the presence of the solid material just at the
junctions or interfaces of the cells, and for the forces by which
it is confined to, and moulded to the form of, these intercellular
or interfacial contacts. It is to Dreyer that we chiefly owe the
physical or mechanical theory of spicular conformation which I have
just described,—a theory which ultimately rests on the form assumed,
under surface-tension, by an aggregation of cells or vesicles. But
this fundamental point being granted, we have still several possible
alternatives by which to explain the details of the phenomenon.
Dreyer, if I understand him aright, was content to assume that the
solid material, secreted or excreted by the organism, accumulated
in the interstices between the cells, and was there subjected to
mechanical pressure or constraint as the cells got more and more
crowded together by their own growth and that of the system generally.
As far as the general form of the spicules goes, such explanation is
not inadequate, though under it we may have to renounce some of our
assumptions as to what takes place at the outer surface of the system.
But in all (or most) cases where, but a few years ago, the concepts
of secretion or excretion seemed precise enough, we are now-a-days
inclined to turn to the phenomenon of adsorption as a further stage
towards the elucidation of our facts. Here we have a case in point.
In the tissues of our sponge, wherever two cells meet, there we
have a definite _surface_ of contact, and there accordingly we
have a manifestation of surface-energy; and the concentration of
surface-energy will tend to be a maximum at the _lines_ or edges
whereby the three, or four, such surfaces are conjoined. Of the
micro-chemistry of the sponge-cells our ignorance is great; but
(without venturing on any hypothesis involving the chemical details of
the process) we may safely assert that there is an inherent probability
that certain substances will tend to be concentrated and ultimately
deposited just in these lines of intercellular contact and conjunction.
In other words, adsorptive concentration, under osmotic pressure, at
and in the surface-film which constitutes the mutual boundary between
contiguous {448} cells, emerges as an alternative (and, as it seems
to me, a highly preferable alternative) to Dreyer’s conception of
an accumulation under mechanical pressure in the vacant spaces left
between one cell and another.