TRILOBITE-LIKE ANIMAL; B, DIAGRAM TO ILLUSTRATE THE SUGGESTED FORMATION OF
THE NOTOCHORD FROM A VENTRAL GROOVE; C, DIAGRAM TO ILLUSTRATE THE SUGGESTED
FORMATION OF THE POST-BRANCHIAL GUT BY THE CONTINUATION OF THE SAME PROCESS
OF VENTRAL GROOVE-FORMATION, COMBINED WITH OBLITERATION OF APPENDAGES AND
GROWTH OF PLEURAL FOLDS; D, DIAGRAM TO ILLUSTRATE THE COMPLETION OF THE
VERTEBRATE TYPE BY THE MEETING OF THE PLEURAL FOLDS IN THE MID-VENTRAL LINE
WITH THE OBLITERATION OF THE ATRIAL CAVITY AND THE CONVERSION OF THE
VENTRAL GROOVE INTO THE CLOSED ALIMENTARY CANAL.
_Al._, alimentary canal; _N._, nervous system; _My._, myotome; _Pl._,
pleuron; _App._, appendage; _Neph._, nephrocoele; _Met._, metacoele; _Sd._,
segmental duct; _Mes._, mesonephros; _At._, atrial chamber; _Nc._,
notochord; _H._, heart; _F._, fat body; _Ng._, notochordal groove. (These
diagrams are intended to complete the diagrams on p. 413, which, as stated
there, were purposely left incomplete.)]
{454}Finally, with the conversion of this groove into a tube, the opening
of the oral into the respiratory chamber, and the formation of an atrium by
the ventralwards growth of the pleural folds, the formation of a Vertebrate
was completed (Fig. 167, D).
In my own mind I picture to myself an animal which possessed eurypterid and
trilobite characters combined, in which a notochordal tube had been formed
in the way suggested, and a respiratory chamber which communicated with the
cloaca by means of a grooved channel along the mid-ventral line of the
metasomatic portion of the body. On each side of this channel were the
remains of the metasomatic appendages (pronephric). The whole was enveloped
in the pleural folds, which probably at this time did not yet meet in the
middle line to form a new ventral surface. This respiratory chamber, owing
to the digestive power of the epidermis, assisted in the process of
alimentation to such an extent as to supersede the temporary notochordal
tube, with the effect of bringing about the conversion of the metasomatic
groove into a closed canal, and so the formation of an alimentary tube
continuous with the respiratory chamber. The amalgamation of the pleural
folds ventrally completed the process, and so formed an animal resembling
the Cephalaspidæ, Ammocoetes, or Amphioxus.
I have endeavoured in this chapter to make some suggestions upon the origin
of the notochord and of the vertebrate gut in accordance with my theory of
the origin of vertebrates. I feel, however, strongly that these suggestions
are much more speculative than those put forward in the previous chapters,
and of necessity cannot give the same feeling of soundness as those based
directly upon comparative anatomy and histology. Still, the fact remains
that the origin of the notochord is at present absolutely unknown, and that
my speculation that it may have originated as an accessory digestive tube
is at all events in accordance with the most widely spread opinion that it
arises in close connection with an alimentary canal.
{455}CHAPTER XIV
_THE PRINCIPLES OF EMBRYOLOGY_
The law of recapitulation.--Vindication of this law by the theory
advanced in this book.--The germ-layer theory.--Its present position.--A
physiological not a morphological conception.--New fundamental law
required.--Composition of adult body.--Neuro-epithelial syncytium and
free-living cells.--Meaning of the blastula.--Derivation of the Metazoa
from the Protozoa. Importance of the central nervous system for Ontogeny
as well as for Phylogeny.--Derivation of free-living cells from
germ-cells.--Meaning of coelom.--Formation of neural canal.--Gastrula of
Amphioxus and of Lucifer.--Summary.
In a discussion upon this theory of mine, which took place at Cambridge on
November 25 and December 2, 1895, it was said that such a theory was
absolutely and definitely put out of court, because it contravened the
principles of embryology, was opposed, therefore, to our surest guide in
such matters; and the law was laid down with great assurance that no claim
for genetic relationship between two groups of animals can be allowed which
is based upon topographical and structural coincidences revealed by the
study of the anatomy of two adult animals, however numerous and striking
they may be, if there are fundamental differences in the embryology of the
members of these two groups.
According to my theory the old gut of the arthropod still exists in the
vertebrate as the tubular lining of the central nervous system, and the
vertebrate has formed a new gut. According to the principles of embryology
as held up to the present, in all animals above the Protozoa, the different
structures of the body arise from three definite embryonic layers, the
epiblast, mesoblast, and hypoblast, and in all cases the gut arises from
the hypoblastic layer. In the vertebrate the gut also arises from the
hypoblast, while the neural canal is epiblastic. My theory, then, makes the
impossible assertion that what was hypoblast in the arthropod has become
epiblast in the vertebrate, and what was epiblast in the arthropod has
become hypoblast in the vertebrate. Such a conception is supposed to be so
{456}absolutely impossible that it only requires to be stated to be
dismissed as an absurdity.
Against this opinion I claim boldly that my theory is not only not contrary
to the principles of embryology, but is mainly based upon the teachings of
embryology. I wish here not to be misunderstood. The great value of the
study of embryology for questions of the sequence of the evolution of
animals is to be found in what is known as the Law of Recapitulation, which
asserts that every animal gives some indication in the stages of its
individual development of its ancestral history. Naturally enough it cannot
pass through all the stages of its past history with equal clearness, for
what has taken millions of years to be evolved has to be compressed into an
evolution lasting only a few months or weeks, or even less.
When in the highest vertebrate a vestigial organ, such as the pineal gland,
can be traced back without leaving the vertebrate kingdom to a distinct
median eye, such as is found in the lamprey, that rudimentary organ is
evidence of an organ which was functional in the earliest vertebrates or
their immediate ancestors. So it is generally with well defined vestigial
organs found in the adult animal; they always indicate an organ which was
functional in the near ancestor.
Passing from the adult to the embryo we still find the same law. Here,
also, vestigial organs are met with, which may leave no trace in the adult,
but indicate organs which were functional in the near ancestor. Thus, but
for embryology, we should have no certainty that the air-breathing
vertebrates had been derived from water-breathing fishes; the indication is
not given by any close resemblance between the formation of the embryos in
their earliest stages, but by the formation of vestigial gill-arches even
in the embryos of the highest mammal.
For all questions of evolution the presence of vestigial organs in the
embryo is the important consideration, for they give an indication of near
ancestry; the early formation of the embryo concerns a much more remote
ancestral period, all vestigial organs of which may well have been lost and
obscured by coenogenetic changes. Let us, then, consider the two
things--the vestigial organs and the early formation of the
embryo--separately, and see how far my opponents are justified in their
statement that my theory contravenes the principles of embryology.
{457}First, I will take the teachings of vestigial organs and the
arrangement of organs found in the vertebrate embryo. Here it is impossible
to say that my theory is contrary to the teaching of embryology, for as the
previous chapters have shown again and again, the argument is based very
largely upon the facts of embryology. In the first place, the comparison
which I have chiefly made is a comparison between the larval form of a very
low vertebrate and the arthropod group, a comparison which exists only for
the larval form, and not for the adult. The whole theory, then, is based
upon a developmental stage of the vertebrate, and not upon the anatomy of
the adult.
Throughout the whole history it seems to me perfectly marvellous how
completely the law of recapitulation is vindicated by my theory of the
origin of the vertebrate. The theory asserts that the clue to the origin of
vertebrates is to be found in the tubular nature of the central nervous
system of the vertebrate; in that the vertebrate central nervous system is
in reality formed of two things: (1) a central nervous system of the
arthropod type, and (2) an epithelial tube in the position of the
alimentary canal of the arthropod.
Is it possible for embryology to recapitulate such a phylogenetic history
more clearly than is here the case? In order to avoid all possibility of
our mistaking the clue, the nerve-tube in the embryo always opens into the
anus at its posterior end, while in the larval Amphioxus it is actually
still open to the exterior at the anterior end. The separateness of the
tube from the nervous system at its first origin is shown especially well
in the frog, where, as Assheton has pointed out, owing to the pigment in
the cells of the external layer of epithelium, a pigmented tube is formed,
on the outside of which the nervous tissue is lying, and step by step the
gradual intermingling of the nerve-cells and the pigmented lining cells can
be followed out.
Consider the shape of the nerve-tube when first formed in the vertebrate.
At the cephalic end a simple bulged-out tube with two simple anterior
diverticula, which passes into a narrow straight spinal tube; from this
large cephalic bulging a narrow diverticulum, the infundibulum, passes to
the ventral surface of the forming brain. This tube is the embryological
expression of the simple dilated cephalic stomach, with its ventral
oesophagus and two anterior diverticula, which opens into the straight
intestine of the arthropod. Nay, more, by its very shape, and the
invariable presence of two anterior {458}diverticula, it points not only to
an arthropod ancestry, but to a descent from a particular group of
primitive arthropods. Then comes the formation of the cerebral vesicles,
and the formation of the optic cup, telling us as plainly as can be how the
invasion of nervous material over this simple cephalic stomach and its
diverticula has altered the shape of the original tube, and more and more
enclosed it with nervous elements.
So, too, in the spinal cord region. When the tube is first formed, it is a
large tube, the latero-ventral part of which presents two marked bulgings;
connecting these two bulgings is the anterior commissure. These two lateral
bulgings, with their transverse commissure, represent, with marked
fidelity, the ventral ganglion-masses of the arthropod with their
transverse commissure, and occupy the same position with respect to the
spinal tube, as the ganglion-masses do with respect to the intestine in the
arthropod. Then the further development shows how, by the subsequent growth
of the nervous material, the calibre of the tube is diminished in size, and
the spinal cord is formed.
Again, I say, is it possible to conceive that embryology should indicate
the nature of the origin of the vertebrate nervous system more clearly than
it does?
It is the same with all the other organs. Take, for instance, the skeletal
tissues. The study of the vertebrate embryo asserts that the cartilaginous
skeleton arose as simple branchial bars and a simple cranio-facial
skeleton, and also that the parenchymatous variety of cartilage represents
the embryonic form. Word for word, the early embryonic stage of the
vertebrate skeleton closely resembles the stage reached in the arthropod,
as shown by Limulus, and again records, unmistakably, the past history of
the vertebrate.
So, too, with the whole of the prosomatic region; the situation of the old
mouth, the manner in which the nose of the cephalaspidian fishes arose from
the palæostracan, are all shown with vivid clearness by Kupffer's
investigations of the early stage of Ammocoetes, while at the same time the
closure of the oral cavity by the septum shows how the oral chamber was
originally bounded by the operculum. Nay, further, the very formation of
this chamber embryologically was brought about by the forward growth of the
lower lip, just as it must have been if the chilaria grew forward to form
the metastoma.
So, too, the study of the embryo teaches that the branchiæ arise as
{459}ingrowths, that the heart arises as two longitudinal veins, just as
the theory supposes from the facts provided by Limulus and the scorpions.
No indication of the origin of the thyroid gland is given by the study of
its structure in any adult vertebrate, but in the larval form of the
lamprey there is still preserved for us a most graphic record of its past
history.
The close comparisons which it is possible to make between the eye-muscles
of the vertebrate and the recti muscles of the scorpion group on the one
hand, and between the pituitary and coxal glands on the other, are based
upon, or at all events are strikingly confirmed by, the study of the
coelomic cavities and the origin of these muscles in the two groups. In
fact the embryological evidence of the double segmentation in the head and
the whole nature of the cranial segments is one of the main
foundation-stones on which the whole of my theory rests.
So it is throughout. Turn to the excretory organs--it is not the kidney of
the adult animal which leads direct to the excretory organs of the
primitive arthropod, but the early embryonic origin of that kidney.
So far from having put forward a theory which runs counter to the
principles of embryology, I claim to have vindicated the great Law of
Recapitulation which is the foundation-stone of embryological principles.
My theory is largely based upon embryological facts, and its strength
consists in the manner in which it links together into one harmonious
whole, the facts of Embryology, Palæontology, Anatomy, and Physiology. Why,
then, is it possible to assert that my theory disregards the principles of
embryology, when, as we have seen, embryology is proclaiming as loudly as
possible how the vertebrate arose? In my opinion, it is because the
embryologists have to a large extent gone wrong in their fundamental
principles, and have attached more weight to these faulty fundamental
principles than to the obvious facts which, looked at thoughtfully, could
not have failed to suggest a doubt as to the correctness of these
'principles.'
The current laws of embryology upon which such weight is laid are based on
the homology of the germinal layers in all Metazoa, and state that in all
cases after segmentation is finished a blastula is formed, from which there
arises a gastrula, formed of an internal layer, the hypoblast, and an
external layer, the epiblast; subsequently {460}between these arises a
third layer, the mesoblast. These layers are strictly morphological
conceptions, and are stated to be homologous in all cases, so that the
hypoblast of one animal must be homologous to the hypoblast of another. In
order, therefore, to compare two adult animals for the purpose of finding
kinship between them, it is necessary to find whether parts such as the
gut, which in both cases have the same function, arise from the same
germinal layer in the embryo. We can, in fact, have no certainty of
kinship, even although the two animals are built up as far as the adult
state is concerned on a remarkably similar plan, unless we can study their
respective embryos and find out what parts arise from the hypoblast and
what from the epiblast. The homology of the germinal layers constitutes in
all cases of disputed relationship the court of final appeal. A new gut,
therefore, in any animal can only be formed from hypoblast, and any theory,
such as that advocated in this book, which deals with the formation of a
new gut, and does not form that gut from pre-existing hypoblast, must of
necessity be wrong and needs no further consideration.
Such is the result of current conceptions--conceptions which to be valid
must be based upon an absolutely clear morphological definition of the
formation of the germinal layers, a definition not based on their
subsequent history and function, but determined solely by the uniformity of
the manner of their origin.
What, then, is a germinal layer? How can we identify it when it first
arises? What is the morphological criterion by which hypoblast can be
distinguished from epiblast, or mesoblast from either?
This is the question put by Braem, in an admirable series of articles in
the _Biologisches Centralblatt_, and is one that must be answered by every
worker who bases his views of the process of evolution upon embryological
investigation. As Braem points out, the germinal layers are definable
either from a morphological or physiological standpoint. In the one case
they must arise throughout on the same plan, and whatever be their fate in
the adult, they must form at an early stage structures strictly homologous
in all animals. In the other case the criterion is based on function, and
the hypoblast, for instance, is that layer which is found afterwards to
form the definitive alimentary canal. There is no longer any morphological
homology; such layers are analogous; they may be, but are not necessarily,
homologous. Braem gives a sketch of the history of the views held on
{461}the germinal layers, and shows how they were originally a purely
physiological conception, and how gradually such conception changed into a
morphological one, with the result that what had up to that time been
looked upon as analogous structures became strictly homologous and of
fundamental importance in deciding the position of any animal in the whole
animal series.
This change of opinion was especially due to the lively imagination of
Haeckel, who taught that the germinal layers of all Metazoa must be
strictly homologous, because they were all derived from a common ancestral
stock, represented by a hypothetical animal to which he gave the name
Gastræa; an animal which was formed by the simple invagination of a part of
the blastula, thus giving rise to the original hypoblast and epiblast, and
he taught that throughout the animal kingdom the germinal layers were
formed by such an invagination of a part of the blastula to form a simple
gastrula. If further investigation had borne out Haeckel's idea, if
therefore the hypoblast was in all cases formed as the invagination of a
part of a single-layered blastula, then indeed the dogma of the homology of
the germinal layers would be on so firm a foundation that no speculation
which ran counter to it could be expected to receive acceptance; but that
is just what has not taken place. The formation of the gastrula by simple
invagination of the single-layered blastula is the exception, not the rule,
and, as pointed out by Braem, is significantly absent in the earliest
Metazoa; in those very places where, on the Gastræa theory, it ought to be
most conspicuous.
Braem discusses the question most ably, and shows again and again that in
every case the true criterion upon which it is decided whether certain
cells are hypoblastic or not is not morphological but physiological. The
decision does not rest upon the answer to the question, Are these cells in
reality the invaginated cells of a single-celled blastula? but to the
question, Do these cells ultimately form the definitive alimentary canal?
The decision is always based on the function of the cells, not on their
morphological position. Not only in Braem's paper, but elsewhere, we see
that in recent years the physiological criterion is becoming more and more
accepted by morphologists. Thus Graham Kerr, in his paper on the
development of Lepidosiren, says: "It seems to me quite impossible to
define a layer as hypoblastic except by asking one or other of the two
questions: (1) Does it form the lining of an archenteric cavity? and (2)
{462}Does it become a certain part of the definitive epithelial lining of
the gut?"
The appearance of Braem's paper was followed by a criticism from the pen of
Samassa, who agrees largely with Braem, but thinks that he presses the
physiological argument too far. He considers that morphological laws must
exist for the individual development as well as for the phylogenetic, and
finishes his article with the following sentence, a sentence in which it
appears to me he expresses what is fast becoming the prevailing view: "Mit
dem Satz, den man mitunter lesen kann: 'es muss doch auch für die Ontogenie
allgemeine Gesetze geben' kann leicht Missbrauch getrieben werden; diese
allgemeinen Gesetze giebt es wohl, aber sie liegen nicht auf flacher Hand
und bis zu ihrer Erkenntnis hat es noch gute Wege; das eine kann man aber
wohl heute schon sagen, die Keimblätterlehre gehört zu diesen allgemeinen
Gesetzen nicht."
I conclude, then, that we ought to go back to a time previous to that of
Haeckel and ask ourselves seriously the question, When we lay stress on the
germinal layers and speak of this or that organ arising from this or that
germinal layer, are we thereby adding anything to the knowledge that we
already possess from the study of the anatomy and physiology of the adult
body? If by hypoblast we only mean the internal surface or alimentary canal
and its glands, etc., and by epiblast we mean the external surface or skin
and its glands, etc., while mesoblast indicates the middle structures
between the other two, then I fail to see what advantages we obtain by
using Greek terms to express in the embryo what we express in English in
the adult.
The evidence given by Braem, and it could be strengthened considerably, is
conclusive against the morphological importance of the theory of the
germinal layers, and transfers the fundamental importance of the early
embryonic formation, from that of a three-layered embryo to that of a
single-layered embryo--the blastula--from which, in various ways, the adult
animal has arisen.
The derivation of both arthropod and vertebrate from such a single-layered
animal is perfectly conceivable, even though the gut of the latter is not
homologous with the gut of the former. We have seen that the teachings of
embryology, as far as its later stages are concerned, afford one of the
main supports upon which this theory rests. What, therefore, is required to
complete the story is the way {463}in which these later stages arise from
the blastula stage; here, as in all cases, the ontogenetic laws must be in
harmony with the phylogenetic; of the latter the most important is the
steady development of the central nervous system for the upward progress of
the animal race. The study of comparative anatomy indicates the central
nervous system, not the gut, as the keystone of the edifice. So, also, it
must be with ontogeny; here also the central factor in the formation of the
adult from the blastula ought to be the formation of the central nervous
system, not that of the gut.
Such, it appears to me, is the case, as may be seen from the following
considerations.
The study of the development of any animal can be treated in two ways:
either we can trace back from the adult to the very beginning in the ovum,
or we can trace forward from the fertilized egg to the adult. Both methods
ought to lead to the same result; the difference is, that in the first case
we are passing from the more known to the less known, and are expressing
the unknown in terms of the known. In the second case we are passing from
the less known to the more known, and are expressing the known in
speculative terms, invented to explain the unknown. What has just been said
with respect to the germinal layers means that, however much we may study
the embryo and try to express the adult in terms of it, we finally come
back to the first way of looking at the question, and, starting with the
adult, trace the continuity of function back to the first formation of
cells having a separate function.
Let us, then, apply this throughout, and see what are the logical results
of tracing back the various organs and tissues from the adult to the
embryo.
The adult body is built up of different kinds of tissues, which fall
naturally, from the standpoint of physiology, into groups. Such groups are,
in the first place--
1. All those tissues which are connected with the central nervous system,
including in that group the nervous system itself.
2. All those tissues which have no connection with the nervous system.
In the second group the physiologist places all germinal cells, all blood-
and lymph-corpuscles, all plasma-cells and connective tissue and its
derivatives--in fact, all free-living cells, whether in a free state or in
a quiescent, so to speak encysted, condition, such as is {464}found in
connective tissue. In the first group the physiologist recognizes that the
central nervous system is connected with all muscular tissues, whether
striped or unstriped, somatic or splanchnic, and that such connection is of
an intimate character. Further, all epithelial cells, either of the outer
or inner surfaces, whether forming special sense-organs and glands, such as
the digestive and sweat-glands, or not, are connected with the nervous
system. Besides these structures, there is another set of organs as to
which we cannot speak definitely at present, which must be considered
separately, viz. all the cells, together with their derived organs, which
line the body-spaces. Whatever may be the ultimate decision as to this
group of cells, it must fall into one or other of the two main groups.
The members of these two groups are so interwoven with one another that
either, if taken alone, would still give the form of the body, so that, in
a certain sense, we can speak of the body as formed of two syncytia,
separate from each other, but interlaced, of which the one forms a
continuous whole by means of cells connected together by a fluid medium or
by solid threads formed in such fluid medium, while the other does not form
a syncytium in the sense that any cell of one kind may be connected with
any cell of another kind, but a syncytium of which all the different
elements are connected together only through the medium of the nervous
system.
If we choose to speak of the body as made up of two syncytia in this way,
we must at the same time recognize the fundamental difference in character
between them. In the one case the elements are connected together only by
what may be called non-living material; there is no direct metabolic
activity caused by the action of one cell over a more distant cell in
consequence of such connection, it is not a true syncytium; in the second
case there is a living connection, the metabolism of one part is directly
influenced by the activity of another, and the whole utility of the system
depends upon such functional connection.
The tissues composing this second syncytium may be spoken of as the
master-tissues of the body, and we may express this conception of the
building up of the body of the higher Metazoa by saying that it is composed
of a syncytial host formed of the master-tissues, which contains within its
meshes a system of free-living cells, none of which have any connection
with the nervous system. This syncytial {465}host is in the adult composed
of a number of double elements, a nerve-cell element, and an epithelial
element, such as muscle-cell, gland-cell, etc., connected together by
nerves; and if such connection is always present as we pass from the adult
to the embryo, if there is no period when, for example, the neural element
exists alone free from the muscle-cell, no period when the two can be seen
to come together and join, then it follows that when the single-layered
blastula stage is reached, muscle-cell and nerve-cell must have fused
together to form a neuro-muscular cell. Similarly with all the other
neuro-epithelial organs; however far apart their two components may be in
the adult, they must come together and fuse in the embryo to form a
neuro-epithelial element.
The close connection between muscle and nerve which has always been
recognized by physiologists, together with the origin of muscle from a
myo-epithelial cell in Hydra and other Coelenterata, led the older
physiologists to accept thoroughly Hensen's views of the neuro-epithelial
origin of all tissues connected with the central nervous system. Of late
years this conception has been largely given up owing to the statement of
His that the nervous system arises from a number of neuroblasts, which are
entirely separate cells, and have at first no connection with muscle-cells
or any peripheral epithelial cells, but subsequently, by the outgrowing of
an axial fibre, find their way to the muscle, etc., and connect with it. I
do not think that His' statement by itself would have induced any
physiologist to give up the conception of the intimate connection of muscle
and nerve, if the work of Golgi, Ramón y Cajal, and others had not brought
into prominence the neurone theory, _i.e._ that each element of the central
nervous system is an independent element, without real connection with any
other element and capable of influencing other cells by contact only. These
two statements, emanating as they did from embryological and anatomical
studies respectively, have done much to put into the background Hensen's
conceptions of the syncytial nature of the motor, neural, and sensory
elements, which make up the master-tissues of the body, and have led to the
view that all the elements of the body are alike, in so far as they are
formed of separate cells each leading an independent existence, without any
real intimate connection with each other.
The further progress of investigation is, it seems to me, bringing us back
to the older conception, for not only has the neuroblast theory {466}proved
very difficult for physiologists to accept, but also Graham Kerr, in his
latest papers on the development of Lepidosiren, has shown that there is
continuity between the nerve-cell and the muscle-cell from the very first
separation of the two sets of elements; in fact, Hensen is right and His
wrong in their respective interpretation of the earliest stages of the
connection between muscle and nerve. So also, it seems to me, the intimate
connection between the metabolism of the gland-cell, as seen in the
submaxillary gland, and the integrity of its nervous connection implies
that the connection between nerve-cell and gland-cell is of the same order
as that between nerve-cell and muscle-cell. Graham Kerr also states in his
paper that from the very commencement there is, he believes, continuity
between nerve-cell and epithelial cell, but so far he has not obtained
sufficiently clear evidence to enable him to speak positively on this
point.
Further, according to the researches of Anderson, the cells of the superior
cervical ganglion in a new-born animal will continue to grow healthily as
long as they remain connected with the periphery, even though entirely
separated from the central nervous system by section of the cervical
sympathetic nerve, and conversely, when separated from the periphery, will
atrophy, even though still connected with the central nervous system. So,
also, on the sensory side, Anderson has shown that the ganglion-cells of
the posterior root-ganglion will grow and remain healthy after separation
of the posterior roots in a new-born animal, but will atrophy if the
peripheral nerve is cut, even though they are still in connection with the
central nervous system. Further, although section of a posterior root in
the new-born animal does not affect the development of the nerve-cells in
the spinal ganglion, and of the nerve-fibres connecting the posterior
root-ganglion with the periphery, it does hinder the development of that
part of the posterior root connected with the spinal ganglion.
These experiments of Anderson are of enormous importance, and force us, it
seems to me, to the same conclusion as that to which he has already
arrived. His words are (p. 511): "I suggest, therefore, that the section of
peripheral nerves checked the development of motor and sensory neurones,
not because it blocked the passage of efferent impulses in the first case
and the reception of stimuli from the periphery in the second, but for the
same reason in both cases, {467}viz. that the lesion disturbed the
chemico-physical equilibrium of an anatomically continuous (neuro-muscular
or neuro-epithelial) chain of cells, by separating the non-nervous from the
nervous, and that the changes occurring in denervated muscle, which I shall
describe later (and possibly those in denervated skin), are in part due to
the reciprocal chemico-physical disturbance effected in these tissues by
their separation from the nervous tissues; also that the section of the
posterior roots checked the development of those portions of them still
attached to the spinal ganglia, because the chemico-physical equilibrium in
those processes is maintained not only by the spinal ganglion-cells, but
also by the intra-spinal cells with which these processes are anatomically
continuous."
What is seen so strikingly in the new-born animal can be seen also in the
adult, and in Anderson's paper references are given to the papers of Lugaro
and others which lead to the same conclusion.
These experiments seem to me distinctly to prove that the connection
between the elements of the peripheral organ and the proximate neurone is
more than one of contact.
We can, however, go further than this, for, apart from the observations of
Apathy, there is direct physiological evidence that the vitality of other
neurones besides the terminal neurones is dependent upon their connection
with the peripheral organ, even though their only connection with the
periphery is by way of the terminal neurone. Thus, as is seen from
Anderson's experiments, section of the cervical sympathetic nerve in a very
young animal causes atrophy of many of the cells in the corresponding
intermedio-lateral tract, cells which I supposed gave origin to all the
vaso-constrictor, pilomotor, and sweat-gland nerves. A still more striking
experiment given by Anderson is the effect of the removal of the periphery
upon the medullation of those efferent fibres which arise from these same
spinal cells, for, as he has shown, section of the nerves from the superior
cervical ganglion to the periphery in a very young animal delays the
medullation in the fibres of the cervical sympathetic--that is, in
preganglionic fibres which are not directly connected with the periphery
but with the terminal neurones in the superior cervical ganglion. So also
on the afferent side a sufficiently extensive removal of sensory field will
cause atrophy of the cells of Clarke's column, so that, just as in the case
of the primary neurones, {468}the secondary neurones show by their
degenerative changes the importance of their connection with the peripheral
organs.
In this way I can conceive the formation of a series of both efferent and
afferent relays in the nervous system by proliferation of the original
neural moiety of the neuro-epithelial elements, every one of which is
dependent upon its connection with the peripheral epithelial elements for
its due vitality, the whole system being a scheme for co-ordination of a
larger and larger number of peripheral elements. Thus the cells of the
vasomotor centre are in connection with the whole system of segmental
vaso-constrictor centres in the lateral horns of the thoracic region of the
cord, so that to cause atrophy of these cells a very extensive removal of
the vascular system would be required. Each of the segmental centres in the
cord supplies a number of sympathetic segments, the connection with all of
which would have to be cut in order to ensure complete removal of the
connection of each of its cells with the periphery, and finally each of the
cells in the sympathetic ganglia supplies a number of peripheral elements,
all of which must be removed to ensure complete severance.
Thus, if we take any arbitrary number, such as 4, to represent the number
of peripheral organ-elements with which each terminal neurone is connected,
and suppose that each neurone has proliferated into sets of 4, then a cell
of the third order, such as a cell of the vasomotor centre, would require
the removal of 64 peripheral elements to cause its complete separation from
the periphery, one of the second order (a cell of the thoracic lateral
horn) 16 elements, one of the first order (a cell of a sympathetic
ganglion) 4 elements.
Such intimate inter-relationship between the neurones, both afferent and
efferent, and their corresponding peripheral organs does not imply that all
nerve-cells are necessarily as closely dependent upon some connection with
the periphery, for just as the proliferation of epithelial or muscle-cells
forms an epithelial or muscular sheet, the elements of which are so
loosely, if at all, connected together that their metabolism is in no way
dependent upon such connection, so also a similar proliferation of the
neural elements may form connections between nerve-cell and nerve-cell of a
similarly loose nature.
It is this kind of proliferation which, in my opinion, would bind together
the separate relays of efferent and afferent neurones, and {469}so give
origin to reflex actions at different levels. Such neurones would not be in
the direct chain of either the afferent or efferent neurones, and so not
directly connected with the periphery, and could therefore be removed
without affecting the vitality of either the efferent or afferent chain of
neurones. In other words, the vitality of the cells on the efferent side
ought not to be dependent on the integrity of the reflex arc. With regard
to the development of the anterior roots, Anderson has shown that this is
the case, for section of all the posterior roots conveying afferent
impulses from the lower limb in a new-born animal does not hinder the
normal development of the anterior roots supplying that limb. Also Mott,
who originally thought that section of all the posterior roots to a limb
caused atrophy of the corresponding anterior roots, has now come to the
same conclusion as other observers, and can find no degeneration on the
efferent side due to removal of afferent impulses.
Again, the process of regeneration after section of a nerve is not in
favour of the neuroblast theory. There is no evidence that the cut end of a
nerve can grow down and attach itself to a muscular or epithelial element
without the assistance of a nerve tube down which to grow. When the cut
nerves connected with the periphery degenerate, that applies only to the
axis-cylinder and the medullary sheath, not to the neurilemma; the
connective tissue elements remain alive and form a tube into which the
growing axon finds its way, and so is conducted to the end-plate or
end-organ of the peripheral structure.
Possibly, as suggested by Mott and Halliburton, the products of
degeneration of the axis-cylinder and medullary sheath stimulate these
connective tissue sheath-cells into active proliferation, and so bring
about the great multiplication of cells arranged as cell-chains, which are
so often erroneously spoken of as forming the young nerves. These
sheath-cells are then supposed to re-form and secrete a pabulum which is
important for the process of regeneration of the down-growing axis-cylinder
and medullary sheath. Without such pabulum regeneration does not take
place, as is seen in the central nervous system, where the sheath of
Schwann is absent.
Again, it is becoming more and more doubtful whether the peripheral
terminations of nerves are ever really free. As far as efferent nerves are
concerned the nervous element may entirely {470}predominate over the
muscular or glandular, as in the formation of the electric organs of the
Torpedo and Malapterurus, but still the final effect is produced by the
alteration of the muscle or gland-cell. On the afferent side especially
free nerve-terminations are largely recognized, or, as in Barker's book,
nerves are spoken of as arising in connective tissue. Thus the numerous
kinds of special sense-organs, such as Pacinian bodies, tendon-organs,
genital corpuscles, etc., are all referred to by Barker under the heading
of "sensory nerve beginnings in mesoblastic tissues." Yet the type of these
organs has been known for a long time in the shape of Grandry's corpuscles
or the tactile corpuscles in the duck's bill, where it has been proved that
the nerve terminates in special large tactile cells derived from the
surface-epithelium.
So also with all the others, further investigation tends to put them all in
the same category, all special sensory organs originating from a localized
patch of surface-epithelium. Thus Anderson has shown me in his specimens
how the young Pacinian body is composed of rows of epithelial cells, into
each of which a twig from the nerve passes. He has also shown me how, in
the case of the tendon-organ, each nerve-fibre passes towards the
attachment of the tendon and then bends back to supply the tendon-organ,
thus indicating, as he suggests, how the nest of epithelial cells has
wandered inwards from the surface to form the tendon-organ. Again,
Meissner's corpuscles and Herbst's corpuscles are evidently referable to
the same class as those of Grandry and Pacini.
Yet another instance of the same kind is to be found in the chromatophores
of the frog and other animals which are under the influence of the central
nervous system and yet have been supposed by various observers to be
pigmented connective tissue cells. The most recent work of Leo Loeb and
others has conclusively shown that such cells are invariably derived from
the surface-epithelium.
Finally, in fishes we find the special sense-organs of the lateral line and
other accessory sensory organs, all of which are indisputably formed from
modified surface epithelial cells.
The whole of this evidence seems to me directly against Barker's
classification of sensory nerve-beginnings in mesoblastic tissues; in none
of these cases are we really dealing with free nervous tissue alone, the
starting point is always a neuro-epithelial couple.
We may then, I would suggest, look upon the adult as formed of {471}a
neural syncytium, which we may call the host, which carries with it in its
meshes a number of free cells not connected with the nervous system. If,
then, we confine our attention to the host and trace back this neural
syncytium to its beginnings in the embryo, we see that, from the very
nature of the neuro-epithelial couple, each epithelial moiety must approach
nearer and nearer to its neural moiety, until at last it merges with it;
the original neuro-epithelial cell results, and we must obtain, as far as
the host is concerned, a single-layered blastula as the origin of all
Metazoa. It follows, further, that there must always be continuity of
growth in the formation of the host, _i.e._ in the formation of the
neuro-epithelial syncytium; that therefore cells which have been previously
free cannot settle down and take part in its formation, as, for instance,
in the case of the formation of any part of the gut-epithelium or of
muscle-cells from free-living cells.
Further, since the neural moiety is the one element common to all the
different factors which constitute the host, it follows that the
convergence of each epithelial moiety to the neural moiety, as we pass from
the adult to the embryo, is a convergence of all outlying parts to the
neural moiety, _i.e._ to the central nervous system, if there is a
concentrated nervous system. Conversely, in the commencing embryo the place
from which the spreading out of cells takes place, _i.e._ from which growth
proceeds, must be the position of the central nervous system, if the
nervous system is concentrated. If the nervous system is diffuse, and forms
a general sub-epithelial layer, then the growth of the embryo would take
place over the whole surface of the blastula.
Turning now to the consideration of the second group of tissues, those that
are not connected with the central nervous system, we find that they
include among them such special cells as the germinal cells, free cells of
markedly phagocytic nature, and cells which were originally free and
phagocytic, but have settled down to form a supporting framework of
connective tissue, and are known as plasma-cells. In the embryo we find
also in many cases free cells in the yolk, forming more or less of a layer,
which function as phagocytes and prepare the pabulum for the fixed cells of
the growing embryo; these cells are known by the name of vitellophags, and
in meroblastic vertebrate eggs form somewhat of a layer known by the name
of periblast. Such cells must be included in the second group, and,
{472}indeed, have been said again and again to give origin to the
free-living blood-corpuscles of the adult. In other cases they are said to
disintegrate after their work is done.
In the adult the free-living lymphocytes and hæmocytes reproduce themselves
from already existing free-living cells, but as we pass back to the embryo
there comes a time, comparatively late in the history of the embryo, when
such free-living cells are not found in the fluids of the body, and they
are said to arise from the proliferation and setting free of cells which
form a lining epithelium. Such formation of leucocytes has been especially
described in connection with the lining epithelium of the coelomic
cavities, as stated in Chapter XII., so that anatomists look upon the
origin of these free cells as being largely from the coelomic epithelium,
or mesothelium, as Minot calls it.
Then, again, the free cells which form the germinal cells can be traced
back to a germinal epithelium, which also is part of the coelom. Thus the
suggestion arises that in the embryo a cellular lining is formed to a
coelomic cavity (mesothelium) composed of cells which have no communication
with the nervous system, and are capable of a separate existence as free
individuals, either in the form of germinal cells or of lymphocytes,
hæmocytes, and plasma-cells, so that these latter free cells may be
considered as living an independent existence in the body, and ministering
to it in the same sense as the germ-cells live an independent existence in
the body. Again, the function of this mesothelium apart from the germ-cell
is essentially excretory and phagocytic. It is the cells of the excretory
organs as well as the lymphocytes which pick up carmine-grains when
injected. It is the cells of the modified excretory organs, as mentioned in
Chapter XII., which, according to Kowalewsky and others, give origin to the
free leucocytes.
We see, then, that the conception of a syncytial neuro-epithelial host
holding in its meshes a number of free cells leads directly to the
questions: What is the coelom? To which category does its lining membrane
belong? and further, also, What is the origin of these free cells?
The Metazoa have been divided into two great groups--those which possess a
coelom (the Coelomata; Lankester's Coelomocoela) and those which do not
(Coelenterata; Lankester's Enterocoela). As an example of the latter we may
take Hydra, because it is a very {473}primitive form, and because its
development has been carefully worked out recently by Brauer.
In Hydra we find a dermal layer of cells and an inner layer of cells
separated by a gelatinous mass known as mesogloea; in this mass between the
dermal and inner layers scattered cells are found, the interstitial cells.
Now, according to Brauer the position of the germ in Hydra is the
interstitial cell-layer. One cell of the ovarium becomes the egg-cell, the
others have their substance changed into yolk-grains, forming the so-called
pseudo-cells, and as such afford pabulum to the growing egg-cell. Thus we
see that in between the dermal and gastral layer of cells a third layer of
cells is found, composed of free living germ-cells, some of which, by the
formation of yolk-granules, become degraded into pabulum for their more
favoured kinsfolk. These interstitial cells are said to arise from the
dermal layer, or ectoderm, but clearly, as in other cases, germ-cells
constitute a class by themselves and cannot be spoken of as originating
from ectoderm-cells or from hypoderm-cells.
So also in Porifera, Minchin states: "In addition to the collared cells of
the gastral layer, and the various cell-elements of the dermal layer, the
body-wall contains numerous wandering cells or amoebocytes, which occur
everywhere among the cells and tissues. Though lodged principally in the
dermal layer, they are not to be regarded as belonging to it, but as
constituting a distinct class of cells by themselves. They are concerned
probably with the functions of nutrition and excretion, and from them arise
the genital products." Further (p. 31): "At certain seasons some of these
cells become germ-cells; hence the wandering cells and the reproductive
cells may be included together under the general term archæocytes." Also
(p. 51): "The mesogloea is the first portion to appear as a structureless
layer between the dermal and gastral epithelia, and is probably a secretion
of the former."
He also points out that in these, the very lowest of the Metazoa, the
separate origin of these archæocytes can be traced back to a very early
period of embryonic life. Thus in _Clathrina blanca_ the ovum undergoes a
regular and total cleavage, resulting in the formation of a hollow ciliated
blastula of oval form. At one point, the future posterior pole of the
larva, are a pair of very large granular cells with vesicular nuclei, which
represent undifferentiated blastomeres and are destined to give rise to the
archæocytes, and, therefore, also to the {474}sexual cells of the adult.
Thus, as he says, from the very earliest period a distinction is made
between the "tissue-forming" cells (my syncytial host) and the archæocytes.
We see, then, that the origin of all these free-living cells can be traced
back to the very earliest of the Metazoa. Here between the dermal and
gastral layers a gelatinous material, the mesogloea is secreted by these
layers. This material is non-living, non-cellular. In it live free cells
which may either be germ-cells, amoebocytes, or 'collencytes' (connective
tissue cells). If this mesogloea were a fluid secretion, then we should
have a tissue of the nature of blood or lymph; if it were solid, then we
should have the foundation of connective tissue, cartilage, and bone.
From this primitive tissue it is easy to see how the special elements of
the vascular, lymphatic, and skeletal tissues gradually arose, the matrix
being provided by the cells of the syncytial host and the cellular elements
by the archæocytes. In fact, we have no right to speak of these lowest
members of the Metazoa as not being triploblastic, as possessing nothing
corresponding to mesoblast, for in these free cells in the mesogloea we
have the origin of the mesenchyme of the higher groups. Thus Lankester,
talking of mesenchyme, says: "I think we are bound to bring into
consideration here the existence in many Coelentera of a tissue resembling
the mesenchyme of Coelomocoela. In Scyphomedusæ, in Ctenophora, and in
Anthozoa, branched fixed and wandering cells are found in the mesogloea
which seem to be the same thing as a good deal of what is distinguished as
mesenchyme in Coelomocoela. These appear to be derived from both the
primitive layers; some produce spicules, others fibrous substance, others
again seem to be amoebocytes with various functions. It appears to be
probable that, though it may be necessary to distinguish other elements in
it, the mesenchyme of Coelomocoela is largely constituted by cells, which
are the mother-cells of the skeletotrophic group of tissues, and are
destined to form connective tissues, blood-vessels, and blood."
Thus we see that the earliest Metazoa were composed of a dermal and gastral
epithelium, with a sub-epithelial nervous system connecting the parts
together, which formed, as it were, a host, carrying around free living
cells of varying function, all of which may be looked on as derived from
archæocytes, _i.e._ germ-cells. From these the coelomatous animals arose,
and here also we find, according to {475}present-day opinion, that the
coelom arose in the first place in the very closest connection with the
germ-cells or gonads. Thus Lankester, in his review of the history of the
coelom, states:--
"The numerous embryological and anatomical researches of the past twenty
years seem to me to definitely establish the conclusion that the coelom is
primarily the cavity, from the walls of which the gonad cells (ova or
spermata) develop, or which forms around those cells. We may suppose the
first coelom to have originated by a closing or shutting off of that
portion of the general archenteron of Enterocoela (Coelentera), in which
the gonads developed as in Aurelia or as in Ctenophora. Or we may suppose
that groups of gonad mother cells, having proliferated from the endoderm,
took up a position between it and the ectoderm, and there acquired a
vesicular arrangement, the cells surrounding the cavity in which liquid
accumulated.
"The coelom is thus essentially and primarily (as first clearly formulated
by Hatschek) the perigonadial cavity or gonocoel, and the lining cells of
gonadial chambers are coelomic epithelium. In some few groups of
Coelomocoela the coeloms have remained small and limited to the character
of gonocoels. This seems to be the case in the Nemertina, the Planarians,
and other Platyhelmia. In some Planarians they are limited in number, and
of individually large size; in others they are numerous."
When Lankester says that "the lining cells of gonadial chambers are
coelomic epithelium," that is equivalent to saying that the lining cells of
the coelom form an epithelium which was originally gonadial, provided that,
as seems to me most probable, his second suggestion, of the coelom being
formed from gonadial mother-cells which have taken up an intermediate
position between endoderm and ectoderm and there acquired a vesicular
arrangement, is the true one. It does not seem to me possible to conceive
of the gonads arising from cells of the epiblast or of the hypoblast, in
the sense that such cells are differentiated cells belonging to a layer
with a definite meaning. When we consider that the gonad gives origin to
the whole of a new individual, that in the protozoan ancestors of the
Metazoa their ultimate aim and object was the formation of gonads, it seems
a wrong conception to speak of the gonads as formed from cells belonging
either to the gut-wall or to the external epithelium. The gonads must stand
in a category by themselves; they represent a whole, {476}while the other
cells represent only a part; they cannot therefore be derived from the
latter. They may, and indeed do, give rise to cells of a subordinate
character, but they cannot rightly be spoken of as derived from such cells.
The very fact mentioned by Lankester, that in the lowest coelomatous
Metazoa, the Platyhelminthes, the coeloms are limited to the character of
simple gonocoels, strongly points to the conclusion that all the coelomic
cells were originally of the nature of gonadial cells, and therefore
free-living and independent of the rest of the cells of the body. Whether
the germ-cells appear, as in Hydra, to be derived from the ectoblast, or,
as is usually stated, from the endoblast, in neither case ought they to be
classed with the internal or external epithelium; they are germ-cells, and
the epithelium which they form is neither epiblastic nor hypoblastic, but
germinal, forming originally a simple gonocoele, afterwards, in the higher
forms, the coelom with its cells of various function. Thus, to quote again
from Lankester, "The coelomic fluid and the coelomic epithelium, as well as
the floating corpuscles derived from that epithelium, acquire special
properties and importance over and above the original functions subservient
to the maturation of the gonadial cells ... the most important developments
of the coelom are in connection with the establishment of an exit for the
generative products through the body-wall to the outer world, and further
in the specialization of parts of its lining epithelium for renal excretory
functions."
Such exits led very early to the formation of coelomoducts, which are true
outgrowths of the coelom itself (p. 14): "The coelomoducts and the
gonocoels of which they are a part, frequently acquire a renal excretory
function, and may retain both the function of genital conduits and of renal
organs, or may, where several pairs are present (metamerized or segmented
animals), subserve the one function in some segments of the body, and the
other function in other segments."
The origin of the coelom and its derivatives from a germinal membrane, as
suggested by Lankester, appears to me most probable, and, if true, it
carries with it conclusions of far-reaching importance, for it necessitates
that all the cells which line true coelomic cavities, and their
derivatives, belong to the category of free-living cells, and are not
connected with the nervous system. The cells in question are essentially
those which line serous cavities and those which form excretory glands such
as the kidneys. In the latter organ we ought especially to be able to
obtain a clear answer to this question, for is {477}it not a gland which
secretes into a duct and might therefore be expected to be innervated in
the same way as other secretory glands? Although there is a strong _primâ
facie_ presumption in favour of the existence of renal secretory nerves,
yet according to the universal opinion of physiologists no evidence in
favour of such nerves has hitherto been found; all the phenomena of
excretion of urine consequent on nerve stimulation are explicable by the
action of nerves on the renal vessels, not on the renal cells. Not only is
the physiological evidence negative up to the present time, but also, I
think, the histological. On the one hand, Retzius has failed to find
nerve-connections with kidney-cells; on the other, Berkley has obtained
such evidence with the Golgi method, but failed entirely with methylene
blue. I do not myself think that the evidence of the Golgi method alone is
sufficient without corroboration by other methods, and, in any case,
Berkley's evidence does not show the nerve-fibres terminating in the
kidney-cells, in the same way as can be shown by modern methods to exist in
the case of epithelial cells of the surface, etc. Quite recently another
paper on this subject has appeared by Smirnow, who appears to have obtained
better results than those given by Berkley.
Apart from these physiological and histological considerations, this
question is also dependent upon the nature of the development of the
excretory organs, for, according to Lankester, all excretory organs may be
divided into the two classes of nephridial organs and coelomostomes, of
which the former are largely derived from epiblast. We should, therefore,
expect to find secretory nerves to nephridial organs, though possibly not
to coelomostomes. The kidneys of the Mammalia are supposed to be true
coelomostomes, although, according to Goodrich's researches, the excretory
organs in Amphioxus are solenocytes, _i.e._ true nephridia.
As to the lining epithelium of the peritoneal, pleural, and pericardial
cavities--_i.e._ the mesothelium--there is no definite evidence that these
cells are provided with nerves. Such surfaces are remarkably insensitive in
the healthy condition, and the pain in such cavities is essentially a
pressure phenomenon and referable to special sense-organs, such as Pacinian
bodies, etc., rather than to the mesothelium itself.
These sense-organs are identical in structure with those in the skin, and,
as Anderson has shown, the nerves of these organs {478}medullate at the
same time as those in the skin, and both obtain their medullary sheaths
earlier than any other nerves, whether afferent or efferent. However
difficult it may be to explain this fact, only one conclusion seems to me
possible--these Pacinian bodies, like the skin Pacinians, originate from a
nest of surface epithelial cells, a conclusion which is extremely probable
on my theory of the origin of vertebrates, but not, as far as I can see, on
any other.
At the present moment the weight of evidence is, to my mind, in favour of
the lining endothelium of the coelomic cavities being composed of free
cells, unconnected with the nervous system rather than the reverse, but I
must confess that the question is undecided. If it be true that the
coelomic lining is partly enterocoelic and partly gonocoelic, as Lankester
teaches, then it would be natural that its cells should be in connection
with the nervous system, to some extent at all events. This view is,
however, based on very slender foundations. If the mesothelium is composed
of cells capable of becoming free, it cannot give rise to the skeletal
muscles, and it cannot therefore be right to speak of the skeletal muscles
as derived from the lining cells of a part of the primary coelom. The
phylogenetic history of the musculature of the different animals points
strongly to its intimate connection with and derivation from surface
epithelial cells rather than from coelomic mesothelial cells. Thus in the
coelenterates, as seen in Hydra, the muscular layer arises directly from a
modification of the surface epithelial cells; and right up to the annelids,
even to the highest form in the Polychæta, we still see it stated that the
musculature, both circular and longitudinal, arises from the ectoderm. In
the Oligochæta and Hirudinea, according to Bergh, there are five rows of
teloblasts on each side, of which four are ectodermic and give rise to the
nerve-ganglia and the circular muscles, while one is mesoblastic and forms
the nephridial organs and the longitudinal muscles. (The latter statement
is, according to Bergh, well known, and is not particularly shown by him.
These longitudinal muscle-bands always lie close against the nervous system
at their first formation, and may well have been derived in connection with
it.)
It is apparently only in the Vertebrata that the lining cells of the
coelomic cavity are definitely stated to give origin to the
body-musculature, and taking into account on the one hand the evidence of
Graham Kerr as to the intimate connection between nerve-cell and
{479}muscle-cell from the very beginning, and on the other the manner in
which all the skeletal muscles of the adult are lined with a lymphatic
endothelium, I am strongly inclined to believe that at the closing up of
the myocoele, when the myomere separates from the mesomere, the lining
cells remain scattered in among the forming muscle-cells and form the
ultimate lymphatic tissue of the muscles. If this is really so, then the
evidence in favour of the mesothelium being composed of free cells not
connected with the nervous system would be much strengthened, for, on the
one hand, an intimate relation exists between the connective tissue cells
and the endothelium of the roots of the lymphatic vessels, a relation
which, according to Virchow, has rendered it impossible to draw any sharp
line of distinction between the two; and, on the other, the lymphatic
endothelium merges into the lining cells of the great serous cavities of
the body.
It is impossible to conceive of an animal possessing a nervous system which
is not in connection with sensory and muscular tissues; an isolated
nerve-cell is a meaningless possession; but it is equally natural to
conceive of a germ-cell being isolated, capable of living an independent
existence. Such a difference between the two kinds of tissues must have
existed from the very commencement of the Metazoa, so that we must, it
seems to me, imagine that in the formation of the Metazoa from the Protozoa
the whole of the body of the latter did not break up into a mass of
separate gonads, each capable of becoming a free-living protozoan similar
to its parent, but that a portion proliferated into a multinucleated
syncytium while the remainder formed the free-living gonads. This
multinucleated syncytium, or host, as it might be called, would still
continue to exist for the purpose of carrying further afield the immortal
gonads, which need no longer be all shed at one time.
In such an animal as _Volvox globator_ we have an indication of the very
kind of animal postulated as connecting the single-celled Protozoa and the
multi-cellular Metazoa, for it consists of a many-celled case which forms a
hollow sphere, each of the cells being provided with flagella for the
purpose of locomotion of the sphere, except a certain number which are not
flagellated; the latter leave the case to swim freely in the fluid
contained within the sphere, and forming spermaries and ovaries, conjugate,
maturate, and then are set free by the rupture of the encircling locomotor
host.
{480}This conception of the predecessors of the Metazoa being composed of a
mortal host, holding within itself the immortal sexual products, leads
naturally to the idea of the separate development of the host from that of
the germ-cells _ab initio_, so that the study of the development of the
Metazoa means the study of two separate constituents of the metazoan
individual--on the one hand, the elaboration of the elements forming the
syncytial host, on the other, of those derived from the free-living
independent germ-cells. The elaboration of the host means the
differentiation of the protoplasm into epithelial, muscular, and nervous
elements, by means of which the gonads were carried further afield and
their nourishment as well as that of the host ensured.
The _rôle_ of the nervous system as the middleman between internal and
external muscular and epithelial surfaces was, I imagine, initiated from
the very earliest time. The further evolution of the host consisted in a
greater and greater differentiation and elaboration of this
neuro-epithelial syncytium, with the result of a steadily increasing
concentration and departmental centralization of the main factor of the
syncytium; in other words, it led to the origin and elaboration of a
central nervous system. In the interstices of this syncytium the gonads
were placed, and at first, doubtless, the life of the host ended when all
the germ-cells had been set free. 'Reproduce and die' was, I imagine, the
law of the Metazoa at its earliest origin, and throughout the ages, during
all the changes of evolution, the reminiscence of such law still manifests
itself even up to the highest forms as yet reached. With the
differentiation of the syncytial host there came also differentiation of
the free-living gonads, so that only some of them attained to the
perfection of independent existence, capable of continuing the species;
while others became subordinate to the first and provided them with
pabulum, manufacturing within themselves yolk-spherules, and thus in the
shape of yolk-cells ministered to the developing egg-cell. Thus arose a
germinal epithelium of which only a few of the elements passed out of the
host as perfect individuals, the remainder being utilized for the nutrition
of these few. Such yolk-cells of the germinal epithelium would still,
however, retain their character as free cells totally independent of the
syncytial host, and, situated as they were between the internal and
external epithelium, capable of amoeboid movement, would naturally have
their phagocytic action {481}utilized either as yolk-cells for the
providing of pabulum to the egg-cell, or as excretory cells for the removal
and rendering harmless of deleterious products of all kinds. Thus the free
cells of the body would become differentiated into the three classes of
germ-cells, yolk-cells, and excretory cells.
Further, the mass of gonads, which originally occupied so large a space
within the interior of the host, necessarily, as the tissues of the host
differentiated more and more, took up less and less space in proportion to
the whole bulk of the host and formed a germinal mass of cells between the
outer and inner epithelial layers. This germinal mass formed an epithelium,
some of the members of which acted as scavengers for the inner and outer
layers of the host, with the result that fluid accumulated between the two
parts of the germinal epithelium in connection respectively with the
external and internal epithelial surfaces of the host, and thus led to the
formation of a gonocoele, which, by obtaining an external opening, a
coelomostome, gave origin to the coelom.
Again, with the longer life of the host, the setting free of the gonads no
longer necessitating the destruction of the host, and also the gonads
themselves requiring a longer and longer time to be fed up to maturity, the
bulk and complexity of the whole organism increased and special supporting
structures became a necessity. The host itself could and did provide these
to a certain extent by secretions from its epithelial elements, but the
intermediate supports were provided by the system of phagocytic cells
utilizing the fluids of the body, at first in the shape of plasma-cells
able to move from place to place, then settling down to form a connective
tissue framework, and, later on, cartilage and bone.
So also were gradually evolved the whole of the endothelial structures; the
lymph-cells, blood-cells, etc., all having their origin from the free cells
of the body, which themselves originated in the extension of a germinal
epithelium. Just as in a bee-hive the egg-cells may form the fully
developed sexual animal, whether drone or queen bee, or the asexual host of
workers, so in the body of the Metazoa the free cells may form either male
or female germ-cells spermatozoa, or ova, or a host of workers, scavengers,
repairers, food-providers, all useful to the community, all showing their
common origin by their absolute independence of the nervous system.
Two points of great importance follow from this method of looking {482}at
the problem. First, the evolution of the animal kingdom means essentially
the evolution of the host, for that is what forms the individual; secondly,
as the host is composed of a syncytium, the common factor of whose elements
is the neural moiety, it follows that the tissue of central importance for
the evolution of the host must be, as indeed it is, the nervous system.
Further, seeing that the growth of the individual means the orderly
spreading out of the epithelial moiety away from the neural moiety, it
follows that the germ-band or germ-area from which growth starts must be in
the position of the nervous system. If then, the nervous system in the
animal is a concentrated one, then the growth will emanate from the
position of such nervous system. If, on the other hand, the nervous system
is diffused, then the growth will also be diffused.
In this book I have throughout argued that the ancestors of vertebrates
belonged to a great group of animals which gave origin also to Limulus and
scorpion-like animals; it is therefore instructive to see what is the
nature of the development of such animals. For this purpose I will take the
development of the scorpion, as given by Brauer, for he has worked out its
development with great thoroughness and care. His papers show that the
segmentation is discoidal, and results in an oval blastodermic area lying
on a large mass of yolk. Very early there separates out in this area
genital cells and yolk-cells, which latter move freely into the yolk and
prepare it into a fluid pabulum for the nutrition of the cells of the
embryonic shield or germ-band. These free yolk-cells do not take part in
the formation of the germinal layers, nor does the endoderm when formed
give origin to free yolk-cells.
The cells of the germ-band form a small compact area, in which by continual
mitosis the cells become more than one-layered, and soon it is found that
those cells which lie close against the fluid pabulum form a continuous
layer and absorb the nutritious material for themselves and the rest of the
embryo. While this area is thus increasing in thickness by continuous
development, the group of genital cells remains always apart, increasing in
number, but being always in a state of isolation from the cells of the rest
of the growing area. Thus from the very first Brauer's observations on the
development of the scorpion point to the formation of a syncytial host
containing separate genital cells. The continuous layer of cells against
the fluid pabulum, which is already functioning as a gut, and may
{483}therefore be called hypoblast, spreads continuously over the yolk, as
also does the surface epithelial layer, or epiblast. Such spreading is
always a continuous one for both surfaces, so that the yolk is gradually
enclosed by a continuous orderly growth from the germ-band, and not by the
settling down of free cells in the yolk here and there to form the
gut-lining. This steady orderly development proceeds owing to the
nourishment afforded by the activity of the free cells or vitellophags and
the absorbing power of the hypoblast, a steady growth round the yolk which
results in the formation of the gut-tube, the outer covering and all the
muscular and excretory organs. Where, then, is this starting-point, this
germ-band from which the whole embryo grows? It forms the mid ventral area
of the adult animal, it corresponds exactly to the position of the central
nervous system. The whole phenomenon of embryonic growth in the scorpion is
exactly what must take place on the argument deduced from the study of the
adult that the animal arises as a neuro-epithelial syncytium, and we see
that that layer of cells which is situated next to the food-material forms
the alimentary tube. It is not a question whether such layer is ventral or
dorsal to the neural cells, but whether it is contiguous to or removed from
the food-material.
Take, again, a meroblastic vertebrate egg as of the bird. Again we find
free cells passing into the yolk to act as vitellophags, the so-called
periblast cells; again we see that the embryo starts from a germ-band or
embryonic shield, and spreads from there continuously and steadily; again
we see that the layer of cells which lies against the yolk absorbs the
fluid pabulum for the growing cells; again we see that the area from which
the whole process of growth starts is that of the central nervous system,
and again we see that those cells which are contiguous to the food form the
commencing gut, and are therefore called hypoblast, though in this case
they are ventral not dorsal to the neural layer.
The comparison of these two processes shows that there is one common
factor, one thing comparable in the two, one thing that is homologous and
is the essential in the formation of that part of the animal which I have
called the host, and that is the central nervous system. Whether the
epithelial layer which lies ventrally to it or the one that is dorsal forms
the gut depends upon the position of the food-mass. Where the food is,
there will be the absorbing layer. {484}Where the food is not, there will
be no gut formation, whatever may have been the previous history of that
layer. If, then, we suppose, as I do, that the vertebrate arose from a
scorpion-like animal without any reversal of dorsal and ventral surfaces,
and that the central nervous system remained the same in the two animals,
then the comparison of the development of the two embryos shows that the
one would be derived from the other if the yolk-mass shifted from the
dorsal to the ventral side of the nervous system. This would leave the
dorsal epithelial layer of the original syncytium free from pabulum; it
would no longer form the definite gut, _but it would still tend to form
itself in the same manner as before, would still grow from a ventrally
situated germ-band dorsalwards to form a tube, would recapitulate its past
history, and show how the alimentary canal of the arthropod became the
neural canal of the vertebrate_. Although this alimentary canal is formed
in the same way as before, it is no longer recognized as homologous with
the scorpion's alimentary canal, but because it no longer absorbs pabulum,
and does not therefore form the definite gut, it is called an epiblastic
tube, and, in the words of Ray Lankester, has no developmental importance.
All the arthropods are built up on the same type, and in all the
development may in its broad outlines be referred to the type just
mentioned. So also with the vertebrate group; in both cases the position of
the central nervous system determines the starting area of embryonic
growth. In both cases the absorbing layer shows the position of the
definite gut. A concentrated nervous system of this type is common to all
the segmented animals from the annelids to the vertebrates, and in all
cases the germ-band which indicates the first formation of the embryo is in
the position of this nervous system.
As far as the embryo is concerned, there is no great difficulty in the
conception that the yolk-mass may have shifted from one side to the other
in passing from the arthropod to the vertebrate, for in the arthropod the
embryo at first is surrounded by yolk and then passes to the periphery of
the egg. If it is permissible to speak of a dorsal and ventral surface to
an egg, and we may imagine the egg held with such dorsal surface uppermost,
then the yolk would be situated ventrally to the embryo, as in the
vertebrate, if the protoplasmic cells of the embryo rose from their central
position to the surface through the yolk, while if they sank through the
yolk, the yolk would be situated dorsally to the embryo, as in the
arthropod.
{485}In cases where there is no yolk, or very little, as in Lucifer and
Amphioxus respectively, the embryo is compelled to feed itself at a very
early age; such embryos form a free-swimming pelagic ciliated blastula, the
invagination of which, for the purpose of collecting food material out of
the open sea, is the simplest method of obtaining nutriment. Here, as in
other cases, it is the physiological necessity which determines the method
of formation of the gut, and such similarity of appearance as exists
between the gastrula of Lucifer and that of Amphioxus, by no means implies
that the gut of the adult Lucifer is homologous with the gut of Amphioxus.
I have compared two meroblastic eggs of the two classes respectively,
because the scorpion's egg is meroblastic. I imagine that no real
difficulty arises with respect to holoblastic eggs, for the experiments of
O. Hertwig and Samassa show that by centrifugalizing, stimulating, and
breaking down of large spheres the holoblastic amphibian egg may be
converted into a meroblastic one, and then development will proceed
regularly, _i.e._ in this case also the growth proceeds from the animal
pole; the large cells of the vegetal pole, like the yolk-cells of the
meroblastic egg, manufacture pabulum for the growing syncytial host.
SUMMARY.
Any attempt to discover how vertebrates arose from invertebrates must be
based upon the study of Comparative Anatomy, of Palæontology, and of
Embryology. The arguments and evidence put forward in the preceding
chapters show most clearly how the theory of the origin of vertebrates
from palæostracans is supported by the geological evidence, by the
anatomical evidence, and by the embryological evidence. Of the three the
latter is the strongest and most conclusive, if it be taken to include
the evidence given by the larval stage of the lamprey.
The stronghold of embryology for questions of this sort is the Law of
Recapitulation, which asserts that the history of the race is
recapitulated to a greater or less extent in the development of the
individual. In the previous chapters such recapitulation has been shown
for all the organs of the vertebrate body. In this respect, then,
embryology has proved of the greatest value in confirming the evidence of
relationship between the palæostracan and the vertebrate, given by
anatomical and geological study.
There is, however, another side to embryology, which claims that the
tissues of all the Metazoa are built up on the same plan; that in all
cases in the very early stage of the embryo three layers are formed, the
epiblast, mesoblast, and hypoblast; that in all animals above the
Protozoa these three layers are {486}homologous, the epiblast in all
cases forming the external or skin-layer, the hypoblast the internal or
gut-layer.
Such a theory, therefore, as is advocated in this book, which turns the
gut of the arthropod into the neural canal of the vertebrate, and makes a
new gut for the vertebrate from the external surface must be wrong, as it
flatly contradicts the fundamental germ-layer theory.
Of recent years grave doubts have been thrown upon the validity of this
theory, doubts which have increased in force year by year as more and
more facts have been discovered which are not in agreement with the
theory. So much is it now discredited that any criticism against my
theory, which is based upon it, weighs nothing in the balance against the
positive evidence of recapitulation already stated. If the germ-layer
theory is no longer credited, upon what fundamental laws is embryology
based?
In this chapter I have ventured to suggest a reply to this question,
based on the uniformity of the laws of growth throughout the existence of
the individual.
In the adult animal the body is composed of two kinds of tissues, those
which are connected with or at all events are under the control of the
nervous system, and those which are capable of leading a free life
independent of the nervous system. These two kinds of tissues can be
traced back from the adult to the embryo, and it is the task of
embryology to find out how these two kinds of tissue originate.
The following out of this line of thought leads to the conception that,
throughout the Metazoa, the body is composed of a host which consists of
the master-tissues of the body, and takes the form of a neuro-epithelial
syncytium, within the meshes of which free living independent organisms
or cells live, so to speak, a symbiotic existence.
The evidence points to the origin of all these free cells from
germ-cells, and thus leads to the conception that the blastula stage of
every embryo represents two kinds of cells, the one which will form the
mortal host being the locomotor neuro-epithelial cell, the other the
independent immortal symbiotic germ-cell. Such conception leads directly
to the conclusion that the blastula stage of every member of the Metazoa
is the embryonic representation of a Protozoan ancestor of the Metazoa;
an ancestor, whose nature may be illustrated by such a living form as
_Volvox globator_, which, like a blastula, is composed of a layer of
cells forming a hollow sphere. These cells partly bear cilia, and so form
a locomotor host, partly are of a different character, and form male and
female germ-cells. The latter leave the surface of the sphere, pass as
free individuals into its fluid contents, form spermaries and ovaries,
and then by the rupture of the mortal locomotor host pass out into the
external medium, as free swimming young Volvox.
It is of interest to note that such members of the Protozoa are among the
most highly developed of the members of this great group.
From such a beginning arose in orderly evolution, on the one hand, all
the neuro-muscular and neuro-epithelial structures of the body--the
so-called master-tissues; on the other, the germ-cells, the
blood-corpuscles, lymph-corpuscles plasma and excretory cells, connective
tissue cells, cartilage and bone-cells, etc., all of them independent of
the central nervous system, all traceable to a modification of the
original germ-cells.
{487}Such a view of the processes of embryology brings embryology into
harmony with comparative anatomy and phylogeny, for it makes the central
nervous system and not the alimentary canal the most important factor in
the development of the host.
The growth of the individual, whether arthropod or vertebrate, spreads
from the position of the central nervous system, regardless of whether
that position is a ventral or dorsal one with respect to the yolk-mass.
Where the pabulum is, there is the definite gut, the lining walls of
which are called in the embryo, hypoblast; but when the pabulum is no
longer there, although a tube is formed in the same manner as the
alimentary canal of the arthropod, it is now called an epiblastic tube,
and is known as the neural tube of the vertebrate.
This is the great fallacy of the germ-layer theory, a fallacy which
consists of an argument in a vicious circle: thus the alimentary canal is
homologous in all of the Metazoa, because it is formed of hypoblast, but
there is no definition of hypoblast, except that it is always that layer
which forms the definitive alimentary canal.
When, after the process of segmentation has been completed, a free
swimming blastula results, unprovided with any store of pabulum in the
shape of yolk, then the same physiological necessity causes such a form
to obtain its nutriment from the surrounding medium. The simplest way to
do this is by a process of invagination, in consequence of which food
particles are swept into the invaginated part and then absorbed. For this
reason in such cases true gastrulæ are formed, as in the case of
Amphioxus among the vertebrates, and Lucifer among the crustaceans; such
a formation does not in the least imply that the gut of the arthropod is
homologous with that of the vertebrate. The resemblance between the two
is not a morphological one, but due to the same physiological necessity.
They are analogous formations, not homologous.
The muscular tissues are found to be formed in close connection with the
nervous tissues, and in very many cases are described as formed from
epiblast, so that there are strong reasons for placing them in a special
category of the so-called mesoblastic tissues. If they be separated out,
then it seems to me, the rest of the mesoblast would consist of the
free-living cells of the body, which are not connected with the central
nervous system. In watching, then, the formation of mesoblast, defined in
this way, we are watching the separation out from the master-tissues of
the body of the independent skeletal and excretory cells.
{488}CHAPTER XV
_FINAL REMARKS_
Problems requiring investigation--
Giant nerve-cells and giant-fibres; their comparison in fishes and in
arthropods; blood- and lymph-corpuscles; nature of the skin; origin of
system of unstriped muscles; origin of the sympathetic nervous system;
biological test of relationship.
Criticism of Balanoglossus theory.--Theory of parallel
development.--Importance of the theory advocated in this book for all
problems of Evolution.
The discussion in the last chapter on the "Principles of Embryology"
completes the evidence which I am able to offer up to the present time in
favour of my theory of the "Origin of Vertebrates." There are various
questions which I have left untouched, but still are well worth discussion,
and may be mentioned here. The first of these is the significance of the
giant nerve-cells and giant nerve-fibres so characteristic of the
brain-region of the lower vertebrates. In most fishes two very large cells
are most conspicuous objects in any transverse section of the _medulla
oblongata_ at the level of entrance of the auditory nerves. Each of these
cells gives off a number of processes, some of which pass in the direction
of the auditory nerves and one very large axis-cylinder process which forms
a giant-fibre, known by the name of a Mauthnerian fibre. Each Mauthnerian
fibre crosses the middle line soon after its origin from the giant-cell,
and passes down the spinal cord on the opposite side right to the tail.
Here, near the end of the spinal cord, it breaks up into smaller fibres,
which are believed by Fritsch and others to pass out directly into the
ventral roots to supply the muscles of the tail. Thus Bela Haller says:
"The Mauthnerian fibres are known to give origin to certain fibres which
supply the ventral roots of the last three spinal nerves, so that their
terminal branches serve, in all probability, for the innervation of the
muscles of the tail-fin." They do not occur in the eel, according to
Haller, or in Silurus, according to Kölliker. {489}Their absence in those
fishes, in which a well-developed tail-fin is also absent, increases the
probability of the truth of Fritsch's original conclusion that these
giant-fibres are associated axis-cylinders for certain definite
co-ordinated movements of the fish, especially for the lateral movement of
the tail.
In Ammocoetes, instead of two Mauthnerian fibres, a number of giant-fibres
are found. They are called Müllerian fibres, and arise from giant-cells
which are divisible into two groups. The first group consists of three
pairs situated headwards of the level of exit of the trigeminal nerves. Two
of these lie in front of the level of exit of the oculomotor nerves, and
one pair is situated at the same level as the origin of the oculomotor
nerves. The second group consists of a number of cells on each side at the
level of the entrance of the fibres of the auditory nerves.
The Müllerian fibres largely decussate, as described by Ahlborn, and then
become the most anterior portion of the white matter of the spinal cord,
forming a group of about eight fibres on each side (Fig. 73). A few fibres
are also found laterally, and slightly dorsally, to the grey matter. These
giant-fibres pass down the spinal cord right to the anal region; their
ultimate destination is unknown. Mayer considers that in the first part of
their course they correspond to those tracts of fibres known as the
"posterior longitudinal bundles" in other vertebrates. I imagine,
therefore, that the spinal part of their course represents the two
antero-lateral descending tracts. The second group of giant-cells, which
appears to have some connection with the auditory nerves, may represent
"Deiter's nucleus." The whole system is probably the central nervous part
of a co-ordination mechanism, which arises entirely in the pro-otic or
prosomatic region of the brain--the great co-ordinating and equilibrating
region _par excellence_.
If we turn now to the arthropod it is a striking coincidence that in the
crayfish and in the lobster the work of Retzius, of Celesia, of Allen, and
of many others demonstrates the existence of an equilibration-mechanism for
the swimming movements of the tail-muscles, which is carried out by means
of giant-fibres. These giant-fibres are the axis-cylinder processes of
giant-cells, situated exclusively in the brain-region, and they run through
the whole ventral ganglionic chain in order to supply the muscles of the
tail. In the ventral nerve-cord of the crayfish, according to Retzius, two
specially large {490}giant-fibres exist, each of which breaks up, at the
last abdominal ganglion, into smaller fibres, which pass directly out with
the nerves to the tail-fin. Allen has shown that, in addition to these two
specially large giant-fibres, there are a number of others, some of which,
similarly to the Müllerian fibres of Ammocoetes, cross the middle line,
while some do not. Each of these arises from a large nerve-cell and passes
to one or other of the last pair of abdominal ganglia. The latter fibres,
he says, send off collaterals, while the two specially large giant-fibres
do not. The cells which give origin to all these large, long fibres are
situated in or in front of the prosomatic region of the brain, similarly to
the giant-cells, which give rise to the corresponding Müllerian fibres of
Ammocoetes. I do not know how far this system is represented in Limulus or
Scorpio.
It is, to my mind, improbable that the Mauthnerian fibres pass out directly
as motor fibres to the muscles of the tail-fin; it is more likely that they
are conducting paths between the equilibration-mechanism in connection with
the VIIIth nerve and the spinal centres for the movements of the tail.
Similarly, with respect to the arthropod, it is difficult to believe that
the motor fibres for the tail-muscles arise in the brain-region. In either
case, the striking coincidence remains that the movements of the tail-end
of the body are regulated by means of giant-fibres which arise from
giant-cells in the head-region of the body in both the Arthropoda and the
lowest members of the Vertebrata.
The meaning of this system of giant-cells and giant-fibres in both classes
of animals is well worthy of further investigation.
Another important piece of comparative work which ought to help in the
elucidation of this problem is the comparison of the blood- and
lymph-corpuscles of the vertebrate with those of the invertebrate groups.
As yet, I have not myself made any observations in this direction, and feel
that it is inadvisable to discuss the results of others until I know more
about the facts from personal observation.
The large and important question of the manner of formation of the
vertebrate skin has only been considered to a slight extent. A much more
thorough investigation requires to be made into the nature of the skin of
the oldest fishes in comparison with the skin of Ammocoetes on the one
side, and of Limulus and the Palæostraca on the other.
The muscular system requires further investigation, not so much {491}the
different systems of the striated voluntary musculature--for these have
been for the most part compared in the two groups of animals in previous
chapters--as the involuntary unstriped musculature, about which no word has
been said. The origin of the different systems of unstriped muscles in the
vertebrate is bound up with the origin of the sympathetic system and its
relation to the cranial and sacral visceral systems. The reason why I have
not included in this book the consideration of the sympathetic nervous
system is on account of the difficulty in finding any such system in
Ammocoetes. Also, so far as I know, the distribution of unstriped muscle in
Ammocoetes has not been worked out.
One clue has arisen quite recently which is of great importance, and must
be worked out in the future, viz. the extraordinary connection which exists
between the action of the sympathetic nervous system and the action of
adrenalin. This substance, which is obtained from the medullary part of the
adrenal or suprarenal glands, when injected into an animal produces the
same effects as stimulation of the nerves, which belong to the
lumbo-thoracic outflow of visceral nerves, _i.e._ the system known as the
sympathetic nervous system, which is distinct from both the cranial and
sacral outflows of visceral nerves. The similarity of its action to
stimulation of nerves is entirely confined to the nerves of this
sympathetic system, and never resembles that of either the cranial or
sacral visceral nerves.
Another most striking fact which confirms the great importance of this
connection between the adrenals and the sympathetic nervous system from the
point of view of the evolution of the latter system is that the extract of
the adrenals always produces the same effect as that of stimulation of the
nerves of the sympathetic system, whatever may be the animal from which the
extract is obtained. Thus adrenalin obtained from the elasmobranch fishes
will produce in the highest mammal all the effects known to occur upon
stimulation of the nerves of its sympathetic system.
Further, the cells, which are always associated with the presence of this
peculiar substance--adrenalin--stain in a characteristic manner in the
presence of chromic salts. In Ammocoetes patches of cells which stain in
this manner have been described in connection with blood-vessels in certain
parts, so that, although I know of no definite evidence of the existence of
cell-groups in Ammocoetes corresponding to the ganglia of the sympathetic
system in other vertebrates, it is {492}possible that further investigation
into the nature and connection of these "chromaffine" cells may afford a
clue to the origin of the sympathetic nervous system. At present it is
premature to discuss the question further.
Finally, another test as to the kinship of two animals of different species
must be considered more fully than I have been able to do up to the present
time. This test is of a totally different nature to any put forth in
previous pages. It is known as the "biological test" of relationship, and
is the outcome of pathological rather than of physiological or anatomical
research. It is possible that this test may prove the most valuable of all.
At present we do not know sufficiently its limitations and its sources of
error, especially in the case of cold-blooded animals, to be able to look
upon it as decisive in a problem of the kind considered in this book.
The nature of this test is as follows: It has been found that the serum of
the blood of another animal, when injected in sufficient quantity into a
rabbit, will cause such a change in the serum of that rabbit's blood that
when it is added to the serum of the other animal a copious precipitate is
formed, although the serum of normal rabbit's blood when mixed with that of
another animal will cause no precipitate whatever. This extraordinary
production of a precipitate in the one case and not in the other indicates
the production of some new substance in the rabbit's serum in consequence
of the introduction of the foreign serum into the rabbit, which brings
about a precipitate when the rabbit's serum containing it is mixed with the
serum originally injected. The barbarous name "antibody" has been used to
express this supposed substance in accordance with the meaning of such a
word as "antitoxin," which has been a long time in use in connection with
preventive remedies against pathogenic bacteria. Now, it is found that the
rabbit's serum containing a particular "antibody" will cause a precipitate
only when added to the serum of the blood of the animal from which the
"antibody" was produced or to the serum of the blood of a nearly related
animal.
Further, if that animal is closely related a precipitate will be formed
nearly as copious as with the original serum, if more distantly related a
cloudiness will occur rather than a precipitate, and if the relationship is
still more distant the mixture of the two sera will remain absolutely
clear. Thus this test demonstrates the close relationship of man to the
anthropoid apes and his more distant {493}relationship to monkeys in
general. By this method very evident blood-relationships have been
demonstrated, especially between members of the Mammalia.
I therefore started upon an investigation into the possibility of proving
relationship in this way between Limulus and Ammocoetes, with the kind
assistance of Mr. Graham Smith. I must confess I was not sanguine of
success, as I thought the distance between Limulus and Ammocoetes was too
great. Dr. Lee, of New York, kindly provided me with most excellent serum
of Limulus, and the first experiments showed that the anti-serum of Limulus
gave a most powerful precipitate with its own serum. Graham Smith then
tried this anti-serum of Limulus with the serum of Ammocoetes, and to his
surprise, and mine, he obtained a distinct cloudiness, indicative of a
relationship between the two animals. This, however, is not considered
sufficient, the reverse experiment must also succeed. I therefore, with
Graham Smith, obtained a considerable amount of blood from the adult
lampreys at Brandon, and produced an anti-serum of Petromyzon, which gave
some precipitate with its own serum, but not a very powerful one. This
anti-serum tried with Limulus gave no result whatever, but at the same time
it gave no result with serum from Ammocoetes, so that the experiment not
only showed that Petromyzon was not related to Limulus, but also was not
related to its own larval form, which is absurd.
Considerable difficulties were encountered in preparing the Petromyzon
anti-serum owing to the extreme toxic character of the lamprey's serum to
the rabbit; in this respect it resembled that of the eel. It is possible
that the failure of the lamprey's anti-serum was due to the necessity of
heating the serum sufficiently to do away with its toxicity before
injecting it into the rabbit. At this point the experiments have been at
present left. It will require a long and careful investigation before it is
possible to speak decisively one way or the other. At present the
experiment is positive to a certain extent, and also negative; but the
latter proves too much, for it proves that the larva is not related to the
adult.
Some day I hope this "biological test" will be of use for determining the
relationships of the Tunicata, the Enteropneusta, Amphioxus, etc., as well
as of Limulus and Ammocoetes.
The origin of Vertebrates from a Palæostracan stock, as put forward in this
book, gives no indication of the systematic position {494}of the Tunicata
or Enteropneusta. Neither the Tunicata nor Amphioxus can by any possibility
be on the direct line of ascent from the invertebrate to the vertebrate.
They must both be looked upon as persistent failures, relics of the time
when the great change to the vertebrate took place. The Enteropneusta are
on a different footing; in their case any evidence of affinity with
vertebrates is very much more doubtful.
The observer Spengel, who has made the most exhaustive study of these
strange forms, rejects _in toto_ any connection with vertebrates, and
considers them rather as aberrant annelids. The so-called evidence of the
tubular central nervous system is worth nothing. There is not the slightest
sign of any tubular nervous system in the least resembling that of the
vertebrate. It is simply that in one place of the collar-region the piece
of skin containing the dorsal nerve of the animal, owing to the formation
of the collar, is folded, and thus forms just at this region a short tube.
My theory explains in a natural manner every portion of the elaborate and
complicated tube of the vertebrate central nervous system. In the
Balanoglossus theory the evolution of the vertebrate tube in all its
details from this collar-fold is simple guesswork, without any reasonable
standpoint. Similarly, the small closed diverticulum of the gut in
Balanoglossus, which is dignified with the name of "notochord," has no
right to the name. As I have already said, it may help to understand why
the notochord has such a peculiar structure, but it gives no help to
understanding the peculiar position of the notochord. The only really
striking resemblance is between the gill-slits of Amphioxus and of the
Enteropneusta. In this comparison there is a very great difficulty, very
similar to that of the original attempts to derive vertebrates from
annelids--the gill-slits open ventrally in the one animal and dorsally in
the other. In both animals an atrial cavity exists which is formed by
pleural folds, and in these pleural folds the gonads are situated so that
the similarity of the two branchial chambers seems at first sight very
complete. In the Enteropneusta, however, there are certain
forms--Ptychodera--in which these pleural folds have not met in the
mid-line in this branchial region, and in these it is plainly visible that
these folds, with their gonads, spring from the ventral mid-line and arch
over the dorsal region of the body. Equally clearly Amphioxus shows that
its pleural folds, with the gonads, spring from the dorsal side of the
animal, {495}and grow ventralwards until they fuse in the ventral mid-line
(_cf._ Fig. 168).
As far, then, as this one single striking similarity between Amphioxus and
the Enteropneusta is concerned it necessitates the reversal of dorsal and
ventral surfaces to bring the two branchial chambers into harmony.
[Illustration: FIG. 168.--DIAGRAM ILLUSTRATING THE POSITION OF THE PLEURAL
FOLDS AND GONADS IN PTYCHODERA (A) AND AMPHIOXUS (B) RESPECTIVELY.
_Al._, alimentary canal; _D.A._, dorsal vessel; _V.A._, ventral vessel;
_g._, gonads; _NC._, notochord; _C.N.S._, central nervous system.]
In a mud-dwelling animal, like Balanoglossus, which possesses no
appendages, no special sense-organs, it seems likely enough that ventral
and dorsal may be terms of no particular meaning, and consequently what is
called ventral in Balanoglossus may correspond to what is dorsal in
Amphioxus; in this way the branchial regions of the two animals may be
closely compared. Such comparison, however, immediately upsets the whole
argument of the vertebrate nature of Balanoglossus based on the relative
position of the central nervous system and gut, for now that part of its
nervous system which is looked upon as the central nervous system in
Balanoglossus is ventral to the gut, just as in a worm-like animal, and not
dorsal to it as in a vertebrate.
There is absolutely no possibility whatever of making such a detailed
comparison between Balanoglossus and any vertebrate, as I have done between
a particular kind of arthropod and Ammocoetes. In the latter case not only
the topographical anatomy of the organs in the two animals is the same, but
the comparison is valid even to microscopical structure. In the former case
the origin of almost all {496}the vertebrate organs is absolutely
hypothetical, no clue is given in Balanoglossus, not even to the segmented
nature of the vertebrate. The same holds good with the evidence from
Embryology and from Palæontology. I have pointed out how strongly the
evidence in both cases confirms that of Comparative Anatomy. In neither
case is the strength of the evidence for Balanoglossus in the slightest
degree comparable. In Embryology an attempt has been made to compare the
origin of the coelom in Amphioxus and in Balanoglossus. In Palæontology
there is nothing, only an assumption that in the Cambrian and Lower
Silurian times a whole series of animals were evolved between Balanoglossus
and the earliest armoured fishes, which have left no trace, although they
were able to hold their own against the dominant Palæostracan race. The
strangeness of this conception is that, when they do appear, they are fully
armoured, as in Pteraspis and Cephalaspis, and it is extremely hard luck
for the believers in the Balanoglossus theory that no intermediate less
armoured forms have been found, especially in consideration of the fact
that the theory of the origin from the Palæostracan does not require such
intermediate forms, but finds that those already discovered exactly fulfil
its requirements.
One difficulty in the way of accepting the theory which I have advocated is
perhaps the existence of the Tunicata. I cannot see that they show any
affinities to the Arthropoda, and yet they are looked upon as allied to the
Vertebrata. I can only conclude that both they and Amphioxus arose late,
after the vertebrate stock had become well established, so that in their
degenerated condition they give indications of their vertebrate ancestry
and not of their more remote arthropod ancestry.
In conclusion, the way in which vertebrates arose on the earth as suggested
in this book carries with it many important far-reaching conclusions with
respect to the whole problem of Evolution.
When the study of Embryology began, great hopes were entertained that by
its means it would be possible to discover the pedigree of every group of
animals, and for this end all the stages of development in all groups of
animals were sought for and, as far as possible, studied. It was soon
found, however, that the interpretation of what was seen was so difficult,
as to give rise to all manner of views, depending upon the idiosyncrasy of
the observer. At his will he decided whether any appearance was
coenogenetic or palingenetic, {497}with the result that, in the minds of
many, embryology has failed to afford the desired clue.
At the same time, the geological record was looked upon as too imperfect to
afford any real help; it was said, and is said, that the Cambrian and
pre-Cambrian periods were so immense, and the animals discovered in the
lower Silurian so highly organized, as to compel us to ascribe the
origination of all the present-day groups to this immense early period, the
animals of which have left no trace of their existence as fossils.
In consequence of, or at all events following upon, the supposed failure of
embryology and of geology to solve the problem of the sequence of evolution
of animal life, a new theory has arisen, which goes very near to the denial
of evolution altogether. This is the theory of parallel development. It
discards the old picture of a genealogical tree with main branches arising
at different heights, these again branching and branching into smaller and
smaller twigs, and substitutes instead the picture of the ribs of a fan,
every rib running independently of every other, each group represented by a
rib reaching its highest development on the circumference of the fan and
coming nearer and nearer to a common point at the handle of the fan. This
point of convergence, where all the groups ultimately meet, is so far back
as to reach to the lowest living organisms.
This, in my opinion, unscientific and inconceivable suggestion has arisen
largely in consequence of a conception which has become firmly fixed in the
minds of very many writers on this subject--the conception that in the
evolution of every group, the higher members of the group are the most
specialized in the peculiarities of that group, and it is impossible to
obtain a new group with different peculiarities from such specialized
members. If, then, a higher group is to arise from a lower, it must arise
from the generalized members of that lower group, in other words, from the
lowest members or those nearly akin to the next lower group.
Similarly, the highest members of this latter group are too specialized,
and again we must go to the more generalized members of the group. In this
way each separate specialized group is put on one side, and so the
conception of parallel development comes into being.
The evidence given in this book dealing with the origin of vertebrates
strikes at the foundations of this belief, for it presents an {498}image of
the sequence of evolution of animal forms in orderly upward progress,
caused by the struggle for existence among the members of the race dominant
at the time, which brought about the origin of the next higher group not
from the lowest members of the dominant group, but from some one of the
higher members of that group.
The great factor in evolution has been throughout the growth of the central
nervous system; from that group of animals which possessed the highest
nervous system evolved up to that time the next higher group must have
arisen.
In this way we can trace without a break, always following out the same
law, the evolution of man from the mammal, the mammal from the reptile, the
reptile from the amphibian, the amphibian from the fish, the fish from the
arthropod, the arthropod from the annelid, and we may be hopeful that the
same law will enable us to arrange in orderly sequence all the groups in
the animal kingdom.
This very same law of the paramount importance of the development of the
central nervous system for all upward progress will, I firmly believe, lead
to the establishment of a new and more fruitful embryology, the leading
feature of which will be, as suggested in the last chapter, not the attempt
to derive from the blastula three germ-layers common to all animals, but
rather two sets of organs--those which are governed by the nervous system
and those which are not--and thus by means of the development of the
central nervous system obtain from embryology surer indications of
relationship than are given at present.
The great law of recapitulation, which asserts that the past history of the
race is indicated more or less in the development of each individual, a law
which of late years has fallen somewhat into disrepute, owing especially to
the difficulty of interpreting the embryological history of the vertebrate,
is triumphantly vindicated by the theory put forward in this book. Each
separate vertebrate organ, one after the other, as shown in the last
chapter, indicates in its development the manner in which it arose from the
corresponding organ of the arthropod. There is no failure in the evidence
of embryology, the failure is in the interpretation thereof.
So, too, my theory vindicates the geological method. There is no failure
here; on the contrary, the record of the rocks proclaims with startling
clearness not only the sequence of evolution in the {499}vertebrate kingdom
itself, but the origin of the vertebrate from the most highly-developed
invertebrate race.
The study of the comparative anatomy of organs down to the finest details
has always been a most important aid in finding out relationship between
animals or groups of animals. My theory endorses this view to the
uttermost, and especially indicates the study of the central nervous system
and its outgoing nerves as that comparative study which is most likely to
afford valuable results.
As for the individual, so for the nation; as for the nation, so for the
race; the law of evolution teaches that in all cases brain-power wins.
Throughout, from the dawn of animal life up to the present day, the
evidence given in this book suggests that the same law has always held. In
all cases, upward progress is associated with a development of the central
nervous system.
The law for the whole animal kingdom is the same as for the individual.
"Success in this world depends upon brains."
{501}BIBLIOGRAPHY AND INDEX OF AUTHORS