WHAT IS TAUGHT BY EMBRYOLOGY?
97. Subject to the qualification expressed in the last chapter,
stimulation of muscles and glands involves a neural process in ingoing
nerve, centre, and outgoing nerve. These are the triple elements of the
“nervous arc.” If muscles were directly exposed to external influences,
they would be stimulated without the intervention of a centre; but as a
matter of fact they never are thus exposed, being always protected by
the skin. Did the skin-nerves pass directly to the muscles underneath,
they would move those muscles, without the intervention of a centre;
but as a matter of fact the skin-nerves pass directly to a centre, so
that it is only _through_ a centre that they can act upon the muscles.
Were muscles and glands directly connected with sensitive surfaces,
their activity would indeed be awakened by direct stimulation; but
unless the muscles were so connected the one with the other, by
anastomosis of fibres or continuity of tissue, that the movement of
one was the movement of all, there would need to be some other channel
by which their separate energies should be combined and co-ordinated.
In the higher organisms anastomosis of muscles is rare, and the
combination is effected by means of the nerves.
98. Although analysis distinguishes the two elements of the
neuro-muscular system, assigning separate properties to the separate
tissues, an interpretation of the phenomena demands a synthesis, so
that a movement is to be conceived as always involving Sensibility,
and a sensation as always involving Motility.[126] In like manner,
although analysis distinguishes the various organs of the body,
assigning separate functions to each, our interpretation demands their
synthesis into an organism; and we have thus to explain how the _whole_
has different _parts_, and how these different parts are brought into
unity. Embryology helps us to complete the fragmentary indications of
Anatomy and Physiology.
99. Take a newly laid egg, weigh it carefully, then hatch it, and when
the chick emerges, weigh both chick and shell: you will find that
there has been no increase of weight. The semifluid contents have
become transformed into bones, muscles, nerves, tendons, feathers,
beak, and claws, all without increase of substance. There has been
_differentiation_ of structure, nothing else. Oxygen has passed into
it from without; carbonic acid has passed out of it. The molecular
agitation of heat has been required for the rearrangements of the
substance. Without oxygen there would have been no development. Without
heat there would have been none. Had the shell been varnished, so as to
prevent the due exchange of oxygen and carbonic acid, no chick would
have been evolved. Had only one part of the shell been varnished, the
embryo would have been deformed.
99_a_. The patient labors of many observers (how patient only those
can conceive who have made such observations!) have detected something
of this wondrous history, and enabled the mind to picture some of the
incessant separations and reunions, chemical and morphological. Each
stage of evolution presents itself as the consequence of a preceding
stage, at once an emergence and a continuance; so that no transposition
of stages is possible; each has its appointed place in the series
(PROBLEM I. § 107). For in truth each stage is a _process_--the sum of
a variety of co-operant conditions. We, looking forward, can foresee in
each what it will become, as we foresee the man in the lineaments of
the infant; but in this prevision we always presuppose that the regular
course of development will proceed unchecked through the regular
succession of special conditions: the infant becomes a man only when
this succession is uninterrupted. Obvious as this seems, it is often
disregarded; and the old metaphysical conception of _potential_ powers
obscures the real significance of Epigenesis. The potentiality of the
cells of the germinal membrane is simply their capability of reaching
successive stages of development under a definite series of co-operant
conditions. We foresee the result, and personify our prevision. But
that result will not take place unless all the precise changes that
are needful serially precede it. A slight pressure in one direction,
insufficient to alter the chemical composition of the tissue, may so
alter its structure as to disturb the regular succession of forms
necessary to the perfect evolution.
100. The egg is at first a microscopic cell, the nucleus of which
divides and subdivides as it grows. The egg becomes a hollow sphere,
the boundary wall of which is a single layer of cells, all so
similar that to any means of appreciation we now possess they are
indistinguishable. They are all the progeny of the original nucleus
and yolk, or cell contents. Very soon, however, they begin to show
distinguishable differences, not perhaps in _kind_, but in _degree_.
The wall of this hollow sphere is rapidly converted into the _germinal
membrane_, out of which the embryo is formed. Kowalewsky (confirmed by
Balfour) has pointed out how in the Amphioxus the hollow sphere first
assumes an oval shape, and then, by an indentation of the under side,
with corresponding curvature of the upper side, presents somewhat
the shape of a bowl. The curvature increases, and the curved ends
approaching each other, the original cavity is reduced to a thin line
separating the upper from the under surface. The cavity of the body is
formed by the curving downwards of this double layer of the germinal
membrane.
101. This is not precisely the course observable in other vertebrates;
but in all, the germinal membrane, which lies like a watch-glass on the
surface of the yolk, is recognizable as two distinct layers of very
similar cells. What do these represent? They are the starting-points
of the two great systems: Instrumental and Alimental. The one yields
the dermal surface; the other the mucous membrane. Each follows an
independent though analogous career. The yolk furnishes nutrient
material to the germinal membrane, and so passes more or less directly
into the tissues; but unlike the germinal membrane, it is not itself
to any great extent the seat of generation by segmentation. There are
two yolks: the yellow and the white (which must not be confounded
with what is called the _white of egg_); and their disposition may be
seen in the diagram (Fig. 14) copied from Foster and Balfour’s work.
The importance of the white yolk is that it passes insensibly into
a distinct layer of the germinal membrane, between the two primary
layers.[127] Each of the three layers of the germinal membrane has its
specific character assigned to it by embryologists, who, however, are
not all in agreement. Some authorities regard the topmost layer as
the origin of the nervous system, the epidermis, with hair, feathers,
nails, horns, the cornea and lens of the eye, etc. To the middle layer
are assigned the muscular and osseous systems, the sexual organs, etc.
To the innermost layer, the alimentary canal, with liver, pancreas,
gastric and enteric glands. Other authorities are in favor of two
primary layers: one for the nervous, muscular, osseous, and dermal
systems; the other for the viscera and unstriped muscles. Between these
two layers, a third gradually forms, which is specially characterized
as the vascular.
[Illustration:
Fig. 14.--_Diagrammatic section of an unincubated hen’s egg._ _bl_,
blastoderm; _w y_, white yolk; _y y_, yellow yolk; _v t_, vitelline
membrane; _x_ and _w_, layers of albumen; _ch l_, chalaza; _a ch_,
air-chamber; _i s m_, internal layer of shell membrane; _s m_,
external layer; _s_, shell.
]
102. Messrs. Foster and Balfour, avoiding the controverted designations
of serous, vascular, and mucous layers, or of sensorial, motor
germinative, and glandular layers, employ designations which are
independent of theoretic interpretation, and simply describe the
position of the layers, namely, _epiblast_ for the upper, _mesoblast_
for the middle, and _hypoblast_ for the under layer. From the epiblast
they derive the epidermis and central nervous system (or would even
limit the latter to the central gray matter), together with some
parts of the sense-organs. From the mesoblast, the muscles, nerves
(and probably white matter of the centres), bones, connective tissue,
and blood-vessels. From the hypoblast, the epithelial lining of the
alimentary canal, trachea, bronchial tubes, as well as the liver,
pancreas, etc.[128] Kölliker’s suggestion is much to the same effect,
namely, that the three layers may be viewed as two epithelial layers,
between which subsequently arises a third, the origin of nerves,
muscles, bones, connective tissue, and vessels.[129]
103. The way in which the history may be epitomized is briefly this:
There are two germinal membranes, respectively representing the
Instrumental and Alimental Systems. Each membrane differentiates, by
different appropriations of the yolk substance, into three primary
layers, _epithelial_, _neural_, and _muscular_. In the epiblast, or
upper membrane, these layers represent: 1°, the future epidermis
with its derivatives--hair, feathers, nails, skin glands, and
chromatophores; 2°, the future nervous tissue; 3°, the future muscular
tissue.[130] (Bone, dermis, connective tissue, and blood-corpuscles are
subsequent formations.)
The hypoblast, or under membrane, in an inverted order presents a
similar arrangement: 1°, the unstriped muscular tissue of viscera and
vessels; 2°, the nervous tissue of the sympathetic system; 3°, the
epithelial lining of the alimentary canal with its glands.
Fundamentally alike as these two membranes are, they have specific
differences; but in both we may represent to ourselves the
_embryological unit_ constituted by an epithelial cell, a nerve-cell,
and a muscle-cell. All the other cells and tissues are adjuncts,
necessary, indeed, to the working of the vital mechanism, but
subordinated to the higher organites.
104. This conception may be compared with that of His in the
division of Archiblast and Parablast assigned by him to the germ
and accessory germ.[131] We can imagine, he says, the whole of the
connective substances removed from the organism, and thus leave behind
a scaffolding in which brain and spinal cord would be the axis,
surrounded by muscles, glands, and epithelium, and nerves as connecting
threads. All these parts stand in more or less direct relation to the
nervous system. All are continuous. By a similar abstraction we can
imagine this organic system removed, and leave behind the connected
scaffolding which is formed from the accessory germ; but this latter
has only _mechanical_ significance; the truly vital functions belong to
the other system.
105. The researches of modern histologists have all converged towards
the conclusion that the organs of Sense are modifications of the
surface, with epithelial cells which on the one side are connected with
terminal hairs, or other elements adapted to the reception of stimuli,
and are connected on the other side through nerve-fibres with the
perceptive centres. It has been shown that nerve-fibres often terminate
in (or among) epithelial cells--sensory fibres at the surface, and
motor-fibres in the glands.[132] Whether the fibres actually penetrate
the substance of the cell, or not, is still disputed. Enough for
our present purpose to understand that there is a _physiological_
connection between the two, and above all that sensory nerves are
normally stimulated through some epithelial structure or other.
[Illustration:
Fig. 15.--_Transverse section of a Blastoderm incubated for
eighteen hours._ The section passes through the medullary groove,
_m e_. A, epiblast. B, mesoblast. C, hypoblast. _m f_, medullary
fold, _c h_, notochord.
]
106. And this becomes clear when we go back to the earliest indications
of development. Look at Fig. 15, representing a transverse section of
the germinal membranes in a chick after eighteen hours’ incubation.
Here the three layers, A, B, and C, have the aspect of simple cells
very slightly differing among each other. Yet since each layer has
ultimately a progeny which is characteristically distinguishable, we
may speak of each not as what it now is, but what it will become.
Although the most expert embryologist is often unable to distinguish
the embryo of a reptile from that of a bird or of a mammal, at certain
stages of evolution, so closely does the one resemble the other, yet
inasmuch as the embryo of a reptile does not, cannot become a bird, nor
that of a bird a mammal, he is justified in looking forward to what
each will become, and in calling each embryo by its future name. On the
same ground, although we cannot point to any such distinction between
the layers of the blastoderm as I have indicated in the separation
of Instrumental and Alimental Systems, nor specify any characters by
which the cells can be recognized as epithelial, neural, and muscular,
yet a forward glance prefigures these divisions. We know that the
first result of the segmentation of the yolk is the formation of cells
all alike, which in turn grow and subdivide into other cells. We
know that these cells become variously modified both in composition
and structure, and that by such differentiations the simple organism
becomes a complex of organs.
107. But here it is needful to recall a consideration sometimes
disregarded, especially by those who speak of Differentiation as if
it were some magical Formative Principle, quite independent of the
state of the organized substance which is formed. There is a luminous
conception--first announced by Goethe, and subsequently developed by
Milne Edwards--which regards the organism as increasing in power and
complexity by a physiological “division of labor,” very similar to
that division of employments which characterizes the developed social
organism. But the metaphor has sometimes been misleading; it has been
interpreted as indicating that Function creates Organ (see PROBLEM I.
§ 88), and as if Differentiation itself were something more than the
expression of the changes resulting from the introduction of different
elements. In the Social Organism a “division of labor” presupposes
that laborers with their labor-materials are already existing; the
change is one of rearrangement: instead of each laborer employing
his skill in doing many kinds of work, he restricts it to one kind,
which he is then able to do with less loss of time and power. Thus is
social power multiplied without increase of population, and the social
organism becomes more complex by the differentiation of its organs. It
is not precisely thus with the Animal Organism during its evolution.
Indeed to suppose that the differentiation of the germinal membrane
into special tissues and organs takes place by any such division
of employments, is to fall into the ancient error of assuming the
organism to exist _preformed_ in the ovum. The unequivocal teaching of
Epigenesis is that each part is produced out of the elements furnished
by previous parts; and for every differentiation there must be a
difference in composition, structure, or texture--the first condition
being more important than the second, the second more important than
the third. The word protoplasm has almost as wide a generality as
the word animal, and is often used in forgetfulness of its specific
values: the protoplasm of a nerve-cell is not the _same_ as that of
a blood-cell, a muscle-cell, or a connective-tissue cell, any more
than a bee is a butterfly, or a prawn a lobster. No sooner has the
specific character been acquired, no sooner is one organite formed
by differentiation, than there is an absolute barrier against any
transformation of it into any other kind of organite. The nerve-cell,
muscle-cell, and epithelial cell have a common starting-point, and a
community of substance; but the one can no more be transformed into
the other than a mollusc can be transformed into a crustacean. In the
homogeneous cellular mass which subsequently becomes the “vertebral
plates,” a group of cells is very early differentiated: this is the
rudimentary spinal ganglion, which becomes enveloped in a membrane,
and then pursues a widely different course from that of the other
cells surrounding it, so that “the same cell which was formerly an
element of the vertebral plate now becomes a nerve-cell, while its
neighbors become cartilage-cells.”[133] Indeed all the hypotheses
of transformation of tissues by means of Differentiation are as
unscientific as the hypotheses of the transformation of animals. In the
organism, as in the Cosmos, typical forms once attained are retained.
There probably was a time in the history of the animal series when
masses of protoplasm by appropriating different materials from the
surrounding medium were differentiated into organisms more complex and
more powerful than any which existed before. But it is obvious that
from a common starting-point there could have been no variations in
development without the introduction of new elements of composition:
there might have been many modifications of structure, but unless
these facilitated modifications of composition, there could never have
resulted the striking differences observed in animal organisms.[134]
108. To return from this digression, we may liken the three primary
layers of the germinal membranes to the scattered and slightly
different masses of protoplasm out of which the animal kingdom
was developed. In this early stage there are no individualized
organites--no nerve-cells or muscle-cells. They are cells ready to
receive modifications both of composition and structure, appropriating
slightly different elements from the yolk, and according to such
appropriation acquiring different properties. And this is necessarily
so, since the different cells have not exactly the same relation to
the yolk, nor are they in exactly the same relation to the incident
forces which determine the molecular changes. The uppermost layer
(epiblast) under such variations develops into epithelium and central
nerve-tissue; the epithelial cell cannot develop into a nerve-cell, the
two organites are markedly unlike, yet both spring from a common root.
Another modification results in the development of muscle-cells from
the inner layer.
109. Hence we can understand how the surface is sensitive even
in organisms that are without nerve-tissue; and also how even in
the highest organisms there is an intimate blending of epithelial
with neural tissues. The same indication explains the existence
of neuro-muscular cells in the _Hydra_, recorded by Kleinenberg,
and of neuro-muscular fibres in the _Beroë_, by Eimer.[135] In the
simpler organisms the surface is at once protective, sensitive, and
absorbent. It shuts off the animal from the external medium, and thus
individualizes it; at the same time it connects this individual with
the medium; for it is the channel through which the medium acts, both
as food and stimulus. The first morphological change is one whereby
a part of the surface is bent inwards, and forms the lining of the
body’s cavity. Soon there follows such a modification of structure
between the outer and inner surfaces (_ectoderm_ and _endoderm_) that
the one is mainly sensitive and protective, the other mainly protective
and absorbent. The outer surface continues indeed to absorb, but its
part in this function is insignificant compared with that of the inner
surface, which not only absorbs but secretes fluids essential to
assimilation. The inner surface, although sensitive, is subjected to
less various stimulation, and its sensibility is more uniform.
110. The uppermost of the primary layers we have seen to be epithelial;
and we know that the first lines of the central nervous system are laid
there. A depression called the medullary groove is the first indication
of the future cerebro-spinal axis. Some writers--Kölliker, for
instance--regard this medullary groove as continuous with but different
from the epithelial layer; others maintain that it lies underneath the
epithelium, just as we see it in later stages, when the differentiation
between epithelial and nerve cell has taken place. Since no one
disputes the fact that when the groove becomes a closed canal its
lining is epithelial, one of two conclusions is inevitable: either
the cells of the primary layer develop in the two diverse directions,
epithelial and neural; or else epithelial cells can be developed on
the surface of neural cells and out of them. The latter conclusion is
one which, involving the conception of transformation, would seem to
be put out of court. I think, then, we must admit that the under side
of the primary layer of cells becomes differentiated into nerve-cells;
and this is in accordance with the observations of Messrs. Foster and
Balfour.[136]
111. While there is this intimate morphological and physiological
blending of epithelial and neural organites, there is an analogous
relation between neural and muscular organites. As the neural layer
lies under the epithelial, the muscular lies under the neural. The
surface stimulation passes to the centre, and is reflected on the
muscles. Embryology thus teaches why a stimulus from the external
medium must be propagated to a nerve-centre before it reaches the
muscles; and why a stimulus on one part of the surface may set all the
organism in movement, by passing through a centre which co-ordinates
all movements. This, of course, only applies to the higher organisms.
In the simpler structures the sensitive surface is directly continuous
with the motor organs.
It is unnecessary here to pursue this interesting branch of our
subject; nor need we follow the analogous evolution of the second
germinal membrane representing the Alimental System. Our attention
must be given to what is known and inferred respecting the elementary
structure of the nerves and centres, on which mainly the interest of
the psychologist settles, since to him the whole of Physiology is
merged in nerve actions.