GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE H
TO THE CLOSE OF EMBRYONIC LIFE.
_External Epiblast._
The change already alluded to in the previous chapter (p. 317) by which the
external epiblast or epidermis becomes divided into two layers, is
completed before the close of stage L.
In the tail region at this stage three distinct strata may be recognized in
the epidermis. (1) An outer stratum of flattened horny cells, which fuse
together to form an almost continuous membrane. (2) A middle stratum of
irregular partly rounded and partly flattened cells. (3) An internal
stratum of columnar cells, bounded towards the mesoblast by a distinct
basement membrane (Pl. 13, fig. 8), unquestionably pertaining to the
epiblast. This layer is especially thickened in the terminal parts of the
paired fins (Pl. 13, fig. 1). The two former of these strata together
constitute the epidermic layer of the skin, and the latter the mucous
layer.
In the anterior parts of the body during stage L the skin only presents two
distinct strata, viz. an inner somewhat irregular layer of rounded cells,
the mucous layer, and an outer layer of flattened cells (Pl. 13, fig. 8).
The remaining history of the external epiblast, consisting as it does of a
record of the gradual increase in thickness of the epidermic strata, and a
topographical description of its variations in structure and thickness in
different parts, is of no special interest and need not detain us here.
In the late embryonic periods subsequent to stage Q the layers of the skin
cease to be so distinct as at an earlier period, partly owing to the
innermost layer becoming less columnar, and partly to the presence of a
large number of mucous cells, which have by that stage made their
appearance.
I have followed with some care the development of the placoid scales, but
my observations so completely accord with those of Dr O. Hertwig[228], that
it is not necessary to record them. The so-called enamel layer is a simple
product of the thickening and calcification of the basement membrane, and
since this membrane is derived from the mucous layer of the epidermis, the
enamel is clearly to be viewed as an epidermic product. There is no
indication of a gradual conversion of the bases of the columnar cells
forming the mucous layer of the epidermis into enamel prisms, as is
frequently stated to occur in the formation of the enamel of the teeth in
higher Vertebrates.
Footnote 228: _Jenaische Zeitschrift_, Vol. VIII.
_Lateral line._
The lateral line and the nervous structures appended to it have been
recently studied from an embryological point of view by Götte[229] in
Amphibians and by Semper[230] in Elasmobranchii.
Footnote 229: _Entwicklungsgeschichte d. Unke._
Footnote 230: _Urogenitalsystem d. Selachier._ Semper's
_Arbeiten_, Bd. II.
The most important morphological result which these two distinguished
investigators believe themselves to have arrived at is the direct
derivation of the lateral nerve from the ectoderm. On this point there is a
complete accord between them, and Semper especially explains that it is
extremely easy to establish the fact.
As will appear from the sequel, I have not been so fortunate as Semper in
elucidating the origin of the lateral nerve, and my observations bear an
interpretation not in the least in accordance with the views of my
predecessors, though not perhaps quite conclusive against them.
It must be premised that two distinct structures have to be dealt with,
viz. the _lateral line_ formed of modified epidermis, and the _lateral
nerve_ whose origin is in question.
The lateral line is the first of the two to make its appearance, at a stage
slightly subsequent to K, in the form of a linear thickening of the inner
row of cells of the external epiblast, on each side, at the level of the
notochord.
This thickening, in my youngest embryo in which it is found, has but a very
small longitudinal extension, being present through about 10 thin sections
in the last part of the head and first part of the trunk. The thickening,
though short, is very broad, measuring about 0.28 Mm. in transverse
section, and presents no signs of a commencing differentiation of nervous
structures. The large intestinal branch of the vagus can be seen in all the
anterior sections in close proximity to this line, and appears to me to
give off to it posteriorly a small special branch which can be traced
through a few sections, vide Pl. 13, fig. 2, _n.l_. But this branch is not
sufficiently well marked to enable me to be certain of its real character.
In any case the posterior part of the lateral line _is absolutely without
any adjoining nervous structures or traces of such_.
The rudiment of the epidermic part of the lateral line is formed of
specially elongated cells of the mucous layer of the epiblast, but around
the bases of these certain rounder cells of a somewhat curious appearance
are intercalated.
There is between this and my next youngest embryo an unfortunately large
gap with reference to the lateral line, although in almost every other
respect the two embryos might be regarded as belonging to the same stage.
The lateral line in the older embryo extends from the hind part of the head
to a point well behind the anus, and is accompanied by a nerve for at least
two-thirds of its length.
In the foremost section in which it appears the intestinal branch of the
vagus is situated not far from it, _and may be seen at intervals giving off
branches to it_. There is no sign that these are otherwise than perfectly
normal branches of the vagus. Near the level of the last visceral cleft the
intestinal branch of the vagus gives off a fair-sized branch, which from
the first occupies a position close to the lateral line though well within
the mesoblast (Pl. 13, fig. 3_a_, _n.l_). This branch is the lateral nerve,
and though somewhat larger, is otherwise much like the nerve I fancied I
could see originating from the intestinal branch of the vagus during the
previous stage.
It rapidly thins out posteriorly and also approaches closer and closer to
the lateral line. At the front end of the trunk it is quite in contact with
it, and a short way behind this region the cells of the lateral line
arrange themselves in a gable-like form, in the angle of which the nerve is
situated (Pl. 13, figs. 3_b_, and 3_c_). In this position the nerve though
small is still very distinct in all good sections, and is formed of a rod
of protoplasm, with scattered nuclei, in which I could not detect a
distinct indication of cell-areas. The hinder part of the nerve becomes
continually smaller and smaller, without however presenting any indication
of becoming fused with the epiblast, and eventually ceases to be visible
some considerable distance in front of the posterior end of the lateral
line.
The lateral line itself presents some points of not inconsiderable
interest. In the first place, it is very narrow anteriorly and throughout
the greater part of its length, but widens out at its hinder end, and is
widest of all at its termination, which is perfectly abrupt. The following
measurements of it were taken from an embryo belonging to stage L, which
though not quite my second youngest embryo is only slightly older. At its
hinder end it was 0.17 Mm. broad. At a point not far from this it was
0.09 Mm. broad, and anteriorly it was 0.05 Mm. broad. These measurements
clearly shew that the lateral line is broadest at what may be called its
growing-point, a fact which explains its extraordinary breadth in the
anterior part of the body at my first stage, viz. 0.28 Mm., a breadth which
strangely contrasts with the breadth, viz. 0.05 Mm., which it has in the
same part of the body at the present stage.
It still continues to form a linear area of modified epidermis, and has no
segmental characters. Anteriorly it is formed by the cells of the mucous
layer becoming more columnar (Pl. 13 fig. 3_a_). In its middle region the
cells of the mucous layer in it are still simply elongated, but, as has
been said above, have a gable-like arrangement, so as partially to enclose
the nerve (Pl. 13, fig. 3_b_). Nearer the hind end of the trunk a space
appears in it between its columnar cells and the flattened cells of the
outermost layer of the skin (Pl. 13, fig. 3_c_), and this space becomes
posteriorly invested by a very definite layer of cells. The space (Pl. 13,
fig. 3_d_) or lumen has a slit-like section, and is not formed by the
closing in of an originally open groove, but by the formation of a cavity
in the midst of the cells of the lateral line. Its walls are formed by a
layer of columnar cells on the inner side, and flattened cells on the outer
side, both layers however appearing to be derived from the mucous layer of
the epidermis. The outer layer of cells attains its greatest thickness
dorsally.
During stages M, N, O, the lateral nerve gradually passes inwards into the
connective tissue between the dorso-lateral and the ventro-lateral muscles,
and becomes even before the close of stage N completely isolated from the
lateral line.
The growth of the lateral line itself remains for some time almost
stationary; anteriorly the cells retain the gable-like arrangement which
characterised them at an earlier period, but cease to enclose the nerve;
posteriorly the line retains its original more complicated constitution as
a closed canal. In stage O the cells of the anterior part of the line, as
well as those of the posterior, commence to assume a tubular arrangement,
and the lateral line takes the form of a canal. The tubular form is due to
a hollowing out of the lateral line itself and a rearrangement of its
cells. As the lateral line becomes converted into a canal it recedes from
the surface.
In stage P the first indication of segmental apertures to the exterior make
their appearance, vide Pl. 13, fig. 4. The lateral line forms a canal
situated completely below the skin, but at intervals (corresponding with
segments) sends upwards and outwards prolongations towards the exterior.
These prolongations do not during stage P acquire external openings. As is
shewn in my figure, a special area of the inner border of the canal of the
lateral line becomes distinguished by its structure from the remainder.
No account of the lateral line would be complete without some allusion to
the similar sensory structures which have such a wide distribution on the
heads of Elasmobranchii; and this is especially important in the present
instance, owing to the light thrown by a study of their development on the
origin of the nerves which supply the sense-organs of this class. The
so-called mucous canals of the head originate in the same way as does the
lateral line; they are products of the mucous layer of the epidermis. They
eventually form either canals with numerous openings to the exterior, or
isolated tubes with terminal ampulliform dilatations.
I have not definitely determined whether the canal-system of the head
arises in connection with the lateral line, or only eventually becomes so
connected. The important point to be noticed is, that at first no nervous
structures are to be seen in connection with it. In stage O nerves for the
mucous canals make their appearance as delicate branches of the main stems.
These nerve-stems are very much ramified, and their branches have, in a
large number of instances, an obvious tendency towards a particular
sense-organ (Pl. 13, figs. 5 and 6).
I have not during stage O been able to detect a case of direct continuity
between the two. This is, however, established in the succeeding stage P,
in the case of the canals, and the facility with which it may be observed
would probably render the embryo Elasmobranch a very favourable object for
studying the connection between nerves and terminal sense-organs. The nerve
(Pl. 13, fig. 7) dilates somewhat before uniting with the sense-organ, and
the protoplasm of the nerve and the sense-organ become completely fused.
The basement membrane of the skin is not continuous across their point of
junction, and appears to unite with a delicate membrane-like structure,
which invests the termination of the nerve. The ampullæ would seem to
receive their nervous supply somewhat later than the canals, and the
terminal swellings of the nerves supplying them are larger than in the case
of the canals, and the connection between the ampullæ and the nerves not so
clear. In the case of the head, there can for Elasmobranchii be hardly a
question that the nerves which supply the mucous canals grow centrifugally
from the original cranial nerve-stems, and do not originate in a peripheral
manner from the integument.
This is an important point to make certain of in settling any doubtful
features in the nervous supply of the lateral line. Professor Semper[231],
with whom as dealing with Elasmobranchii we are more directly concerned,
makes the following statement: "At the time when at the front end the
lateral nerve has already completely separated itself from the ectoderm,
and is situated amongst the muscles, it still lies in the middle of the
body close to the ectoderm, and at the hind end of the body is not yet
completely segmented off (abgegliedert) from the ectoderm." Although the
last sentence of this quotation may seem to be opposed to my statements,
yet it appears to me probable that Professor Semper has merely seen the
lateral nerve partially enclosed in the ectoderm. This position of the
nerve no doubt affords a _presumption, but only a presumption_, in favour
of a direct origin of the lateral nerve from the ectoderm; but against this
interpretation of it are the following facts:
Footnote 231: _Loc. cit._ p. 398.
(1) That the front part of the lateral line is undoubtedly supplied by
branches which arise in the ordinary way from the intestinal branch of the
vagus; and we should not expect to find part of the lateral line supplied
by nerves which originate in one way, and the remainder supplied by a nerve
having a completely different and abnormal mode of origin.
(2) The growth of the lateral line is quite independent of that of the
lateral nerve: the latter arises subsequently to the lateral line, and, so
far as is shewn by the inconclusive observation of my earliest stage, as an
offshoot from the intestinal branch of the vagus; and though it grows along
at first in close contact with the lateral line, yet it never presents, so
far as I have seen, any indubitable indication of becoming split off from
this, or of fusing with it.
(3) The fact that the cranial representatives of the lateral line are
supplied with nerves which originate in the normal way[232], affords a
strong argument in favour of the lateral line receiving an ordinary
nerve-supply.
Footnote 232: Götte extends his statements about the lateral
nerve to the nerves supplying the mucous canals in the head;
but my observations appear to me, as far as Elasmobranchii are
concerned, nearly conclusive against such a derivation of the
nerves in the head.
Considering all these facts, I am led to the conclusion _that the lateral
nerve in Elasmobranchii arises as a branch of the vagus, and not as a
direct product of the external epiblast_.
An interesting feature about the lateral line and the similar cephalic
structures, is the fact of these being the only sense-organs in
Elasmobranchii which originate entirely from the mucous layer of the
epiblast. This, coupled with the well-known facts about the Amphibian
epiblast, and the fact that the mucous canals are the only sense-organs
which originate subsequently to the distinct differentiation of the
epiblast into mucous and horny layers, goes far to prove[233] that the
mucous layer is to be regarded as the active layer of the epiblast, and
that after this has become differentiated, an organ formed from the
epiblast is always a product of it.
Footnote 233: I believe that Götte, amongst his very numerous
valuable remarks in the _Entwicklungsgeschichte der Unke_, has
put forward a view similar to this, though I cannot put my hand
on the reference.
_Muscle-plates._
The muscle-plates at the close of stage K were flattened angular bodies
with the apex directed forwards, their ventral edge being opposite the
segmental duct, and their dorsal edge on a level with the middle of the
spinal cord. They were composed of two layers, formed for the most part of
columnar cells, but a small part of their splanchnic layer opposite the
notochord had already become differentiated into longitudinal muscles.
During stage L the growth of these plates is very rapid, and their upper
ends extend to the summit of the neural canal, and their lower ones nearly
meet in the median ventral line. The original band of muscles (Pl. 11,
fig. 8, _m.p´_), whose growth was so slow during stages I and K, now
increases with great rapidity, and forms the nucleus of the whole voluntary
muscular system. It extends upwards and downwards by the continuous
conversion of fresh cells of the splanchnic layer into muscle-cells. At the
same time it grows rapidly in thickness, but it requires some little
patience and care to unravel the details of this growth; and it will be
necessary to enter on a slight digression as to the relations of the
muscle-plates to the surrounding connective tissue.
As the muscle-plates grow dorsalwards and ventralwards their ends dive into
the general connective tissue, whose origin has already been described
(Pl. 13, fig. 1). At the same time the connective-tissue cells, which by
this process become situated between the ends of the muscle-plates and the
skin, grow upwards and downwards, and gradually form a complete layer
separating the muscle-plates from the skin. The cells forming the ends of
the muscle-plates retain unaltered their primitive undifferentiated
character, and the separation between them and the surrounding
connective-tissue cells is very marked. This however ceases to be the case
in the parts of the muscle-plates on a level with the notochord and lower
part of the medullary canal; the thinnest sections and most careful
examination are needed to elucidate the changes taking place in this
region. The cells which form the somatic layer of the muscle-plates then
begin to elongate and become converted into muscle-cells, at the same time
that they are increasing in number to meet the rapid demands upon them. One
result of these changes is the loss of the original clearness in the
external boundary between the muscle-plates and the adjoining
connective-tissue cells, which is only in exceptional cases to be seen so
distinctly as it may be in Pl. 13, figs. 1 and 8. Longitudinal horizontal
sections are the most instructive for studying the growth of the muscles,
but transverse sections are also needed. The interpretation of the
transverse ones is however rendered difficult, both by rapid alterations in
the thickness of the connective-tissue layer between the skin and the
muscle-plates (shewn in Pl. 13, fig. 8), and by the angular shape of the
muscle-plates themselves.
A careful study of both longitudinal and transverse sections has enabled me
to satisfy myself of the fact that the cells of the somatic layer of the
protovertebræ, equally with the cells of the splanchnic layer, are
converted into muscle-cells, and some of these are represented in the act
of undergoing this conversion in Pl. 13, fig. 8; but the difficulty of
distinguishing the outline of the somatic layer of the muscle-plates, at
the time its cells become converted into muscle-cells, renders it very
difficult to determine whether any cells of this layer join the surrounding
connective tissue. General considerations certainly lead me to think that
they do not; but my observations do not definitely settle the point.
From these facts it is clear, as was briefly stated in the last chapter,
_that both layers of the muscle-plate are concerned in forming the great
lateral muscle, though the splanchnic layer is converted into muscles very
much sooner than the somatic_[234].
Footnote 234: The difference between Dr Götte's account of the
development of the muscles and my own consists mainly in my
attributing to the somatic layer of the muscle-plates a share
in the formation of the great lateral muscles, which he denies
to it. In an earlier section of this Monograph, pp. 333, 334,
too much stress was unintentionally laid on the divergence of
our views; a divergence which appears to have, in part at
least, arisen, not from our observations being opposed, but
from Dr Götte's having taken the highly differentiated
Bombinator as his type instead of the less differentiated
Elasmobranch.
The remainder of the history of the muscle-plates presents no points of
special interest.
Till the close of stage L, the muscle-plates are not distinctly divided
into dorsal and ventral segments, but this division, which is so
characteristic of the adult, commences to manifest itself during stage M,
and is quite completed in the succeeding stage. It is effected by the
appearance, nearly opposite the lateral line, of a layer of connective
tissue which divides the muscles on each side into a dorso-lateral and
ventro-lateral section. Even during stage O the ends of the muscle-plates
are formed of undifferentiated columnar cells. The peculiar outlines of the
intermuscular septa gradually appear during the later stages of
development, causing the well-known appearances of the muscles in
transverse sections, but require no special notice here.
With reference to the histological features of the development of the
muscle-fibres, I have not pushed my investigations very far. The primitive
cells present the ordinary division, well known since Remak, into a
striated portion and a non-striated portion, and in the latter a nucleus is
to be seen which soon undergoes division and gives rise to several nuclei
in the non-striated part, while the striated part of each cell becomes
divided up into a number of fibrillæ. I have not however determined what
exact relation the original cells hold to the eventual primitive bundles,
or anything with reference to the development of the sarcolemma.
_The Muscles of the Limbs._--These are formed during stage O coincidently
with the cartilaginous skeleton, in the form of two bands of longitudinal
fibres on the dorsal and ventral surfaces of the limbs. Dr Kleinenberg
first called my attention to the fact that he had proved the limb-muscles
in _Lacerta_ to be derived from the muscle-plates. This I at first believed
did not hold good for Elasmobranchii, but have since determined that it
does so. Between stages K and L the muscle-plates grow downwards as far as
the limbs and then turn outwards and grow into them (Pl. 18, fig. 1). Small
portions of several muscle-plates come in this way to be situated in the
limbs, and are very soon segmented off from the remainder of the
muscle-plates. The portions of muscle-plates thus introduced into the limbs
soon lose their original distinctness, and can no longer be recognized in
stage L. There can however be but little doubt that they supply the tissue
for the muscles of the limbs. The muscle-plates themselves after giving off
these buds to the limbs grow downwards, and by stage L cease to shew any
trace of what has occurred (Pl. 13, fig. 1). This fact, coupled with the
late development of the muscles of the limbs (stage O), caused me to fall
into my original error.
_The Vertebral Column and Notochord._
In the previous chapter (p. 325) an account was given of the origin of the
tissue destined to form the vertebral bodies; it merely remains to describe
the changes undergone by this in becoming converted into the permanent
vertebræ.
This subject has already been dealt with by a considerable number of
anatomists, and my investigations coincide in the main with the results of
my predecessors. Especially the researches of Gegenbaur[235] may be singled
out as containing the pith of the whole subject, and my results, while
agreeing in all but minor points with his, do not supplement them to any
very great extent. I cannot do more than confirm Götte's[236] account of
the development of the hæmal arches, and may add that Cartier[237] has
given a good account of the later development of the centra. Under the
circumstances it has not appeared to me to be worth while recording with
great detail my investigations; but I hope to be able to give a somewhat
more complete history of the whole subject than has appeared in any single
previous memoir.
Footnote 235: _Das Kopfskelet d. Selachier_, p. 123.
Footnote 236: _Entwicklungsgeschichte d. Unke_, pp. 433-4.
Footnote 237: _Zeitschrift f. Wiss. Anat._ Bd. XXV.,
Supplement.
At their first appearance the cells destined to form the permanent vertebræ
present the same segmentation as the muscle-plates. This segmentation soon
disappears, and between stages K and L the tissue of the vertebral column
forms a continuous investment of the notochord which cannot be
distinguished from the adjoining connective tissue. Immediately surrounding
the notochord a layer formed of a single row of cells may be observed,
which is not however very distinctly marked[238].
Footnote 238: Vide pp. 356, 357.
During the stage L there appear four special concentrations of mesoblastic
tissue adjoining the notochord, two of them dorsal and two of them ventral.
They are not segmented, and form four ridges seated on the sides of the
notochord. They are united with each other by a delicate layer of tissue,
and constitute the rudiments of the neural and hæmal arches. In
longitudinal sections of stage L special concentrated wedge-shaped masses
of tissue are to be seen between the muscle-plates, which must not be
confused with these rudiments. Immediately around the notochord the
delicate investment of cells previously mentioned, is still present.
The rudiments of the arches increase in size and distinctness in the
succeeding stages, and by stage N have unquestionably assumed the
constitution of embryonic cartilage. In the meantime there has appeared
surrounding the sheath of the notochord a well-marked layer of tissue which
stains deeply with hæmatoxylin, and with the highest power may be observed
to contain flattened nuclei. It is barely thicker than the adjoining
sheath, but is nevertheless the rudiment of the vertebral bodies. Pl. 13,
fig. 9, _vb_. Whence does this layer arise? To this question I cannot give
a quite satisfactory answer. It is natural to conclude that it is derived
from the previously existing mesoblastic investment of the notochord, but
in the case of the vertebral column I have not been able to prove this.
Observations on the base of the brain afford fairly conclusive evidence
that the homologous tissue present there has this origin. Gegenbaur
apparently answers the question of the origin of this layer in the way
suggested above, and gives a figure in support of his conclusion (Pl. XXII.
fig. 3)[239].
Footnote 239: None of my specimens resembles this figure, and
the layer when first formed is in my embryos much thinner than
represented by Gegenbaur, and the histological structure of the
embryonic cartilage is very different from that of the
cartilage in the figures alluded to. Götte's very valuable
researches with reference to the origin of this layer in
Amphibians tend to confirm the view advocated in the text.
The layer of tissue which forms the vertebral bodies rapidly increases in
thickness, and very soon, at a somewhat earlier period than represented in
Gegenbaur's Pl. XXII. fig. 4, a distinct membrane (Kölliker's Membrana
Elastica Externa) may easily be recognized surrounding it and separating it
from the adjoining tissue of the arches. Gegenbaur's figure gives an
excellent representation of the appearance of this layer at the period
under consideration. It is formed of a homogeneous basis containing
elongated concentrically arranged nuclei, and constitutes a uniform
unsegmented investment for the notochord (vide Pl. 13, fig. 10).
The neural and hæmal arches now either cease altogether to be united with
each other by a layer of embryonic cartilage, or else the layer uniting
them is so delicate that it cannot be recognized as true cartilage. They
have moreover by stage P undergone a series of important changes. The
tissue of the neural arches does not any longer form a continuous sheet,
but is divided into (1) a series of arches encircling the spinal cord, and
(2) a basal portion resting on the cartilaginous sheath of the notochord.
There are two arches to each muscle-plate, one continuous with the basal
portion of the arch-tissue and forming the true arch, which springs
opposite the centre of a vertebral body, and the second not so continuous,
which forms what is usually known as the intercalated piece. Between every
pair of true arches the two roots of a single spinal nerve pass out. The
anterior root passes out in front of an intercalated piece and the
posterior behind it[240].
Footnote 240: In the adult Scyllium it is well known that the
posterior root pierces the intercalated cartilage and the
anterior root the true neural arch. This however does not seem
to be the case in the embryo at stage P.
The basal portion of the arch-tissue likewise undergoes differentiation
into a vertebral part continuous with the true arch and formed of hyaline
cartilage, and an intervertebral segment formed of a more fibrous tissue.
The hæmal arches, like the neural arches, become divided into a layer of
tissue adjoining the cartilaginous sheath of the notochord, and processes
springing out from this opposite the centres of the vertebræ. These
processes throughout the region of the trunk in front of the anus pass into
the space between the dorsal and ventral muscles, and are to be regarded as
rudiments of ribs. The tissue with which they are continuous, which is
exactly equivalent to the tissue from which the neural arches originate, is
not truly a part of the rib. In the tail, behind the anus and kidneys, the
cardinal veins fuse to form an unpaired caudal vein below the aorta, and in
this part a fresh series of processes originates on each side from the
hæmal tissue adjoining the cartilaginous sheath of the notochord, and
eventually, by the junction of the processes of the two sides, a canal
which contains the aorta and caudal vein is formed below the notochord.
These processes for a few segments coexist with small ribs (vide Pl. 13,
fig. 10), a fact which shews (1) that they cannot be regarded as modified
ribs, and (2) that the tissue from which they spring is to be viewed as a
kind of general basis for all the hæmal processes which may arise, and is
not specially connected with any one set of processes.
While these changes (all of which are effected during stage P) are taking
place in the arches, the tissue of the vertebral bodies or cartilaginous
investment of the notochord, though much thicker than before, still remains
as a continuous tube whose wall exhibits no segmental differentiations.
It is in stage Q that these differentiations first appear in the vertebral
regions opposite the origin of the neural arches. The outermost part of the
cartilage at these points becomes hyaline and almost undistinguishable in
structure from the tissue of the arches[241]. These patches of hyaline
cartilage grow larger and cause the vertebral parts of the column to
constrict the notochord, whilst the intervertebral parts remain more
passive, but become composed of cells with very little intercellular
substance. Coincidently also with these changes, part of the layer internal
to the hyaline cartilage becomes modified to form a somewhat peculiar
tissue, the intercellular substance of which does not stain, and in which
calcification eventually arises (Pl. 13, fig. 11). The innermost layer
adjoining the notochord retains its primitive fibrous character, and is
distinguishable as a separate layer through both the vertebral and the
intervertebral regions. As a result of these changes a transverse section
through the centre of the vertebral regions now exhibits three successive
rings (vide Pl. 13, fig. 11), an external ring of hyaline cartilage
invested by "the membrana elastica externa" (_m.el_), followed by a ring of
calcifying cartilage, and internal to this a ring of fibrous cartilage,
which adjoins the now slightly constricted notochord. A transverse section
of an intervertebral region shews only a thick outer and thin inner ring of
fibrous cartilage, the latter in contact with the sheath of the
unconstricted notochord.
Footnote 241: A good representation of a longitudinal section
at this stage is given by Cartier (_Zeitschrift f. Wiss.
Zoologie_, Bd. XXV., Supplement Pl. IV. fig. 1), who also gives
a fair description of the succeeding changes of the vertebral
column.
The constriction of the notochord proceeds till in the centre of the
vertebræ it merely forms a fibrous band. The tissue internal to the
calcifying cartilage then becomes hyaline, so that there is formed in the
centre of each vertebral body a ring of hyaline cartilage immediately
surrounding the fibrous band which connects the two unconstricted segments
of the notochord. The intervertebral tissue becomes more and more fibrous.
In Cartier's paper before quoted there is a figure (fig. 3) which
represents the appearance presented by a longitudinal section of the
vertebral column at this stage.
The relation of the vertebral bodies to the arches requires a short notice.
The vertebral hyaline cartilage becomes almost precisely similar to the
tissue of the arches, and the result is, that were it not for the "membrana
elastica externa" it would be hardly possible to distinguish the limits of
the two tissues. This membrane however persists till the hyaline cartilage
has become a very thick layer (Pl. 13, fig. 11), but I have failed to
detect it in the adult, so that I cannot there clearly distinguish the
arches from the body of the vertebræ. From a comparison however of the
adult with the embryo, it is clear that the arches at most form but a small
part of what is usually spoken of as the body of the vertebræ.
The changes in the notochord itself during the stages subsequent to K are
not of great importance. The central part retains for some time its
previous structure, being formed of large vacuolated cells with an
occasional triangular patch of protoplasm containing the starved nucleus
and invested by indurated layers of protoplasm. These indurated layers are
all fused, and are probably rightly regarded by Gegenbaur and Götte as
representing a sparse intercellular matter. The external protoplasmic layer
of the notochord ceases shortly after stage K to exhibit any traces of a
division into separate cells, but forms a continuous layer with irregular
prominences and numerous nuclei (Pl. 13, fig. 9). In the stages subsequent
to P further changes take place in the notochord: the remains of the cells
become more scanty and the intercellular tissue assumes a radiating
arrangement, giving to sections of the notochord the appearance of a number
of lines radiating from the centre to the periphery (Pl. 13, fig. 11).
The sheath of the notochord at first grows in thickness, and during stage L
there is no difficulty in seeing in it the fine radial markings already
noticed by Müller[242] and Gegenbaur[243], and regarded by them as
indicating pores. Closely investing the sheath of the notochord there is to
be seen a distinct membrane, which, though as a rule closely adherent to
the sheath, in some examples separates itself from it. It is perhaps the
membrane identified by W. Müller[244] (though not by Gegenbaur) as
Kölliker's "membrana elastica interna." After the formation of the
cartilaginous investment of the notochord, this membrane becomes more
difficult to see than in the earlier stage, though I still fancy that I
have been able to detect it. The sheath of notochord also appears to me to
become thinner, and its radial striation is certainly less easy to
detect[245].
Footnote 242: _Jenaische Zeitschrift_, Vol. VI.
Footnote 243: _Loc. cit._
Footnote 244: _Loc. cit._
Footnote 245: Gegenbaur makes the reserve statement with
reference to the sheath of the notochord. For my own sections
the statement in the text certainly holds good. Fortunately the
point is one of no importance.
EXPLANATION OF PLATE 13.
COMPLETE LIST OF REFERENCE LETTERS.
_al._ Alimentary tract. _ao._ Aorta. _c._ Connective tissue. _cav._
Cardinal vein. _ch._ Notochord. _ep._ Epiblast. _ha._ Hæmal arch. _l._
Liver. _ll._ Lateral line. _mc._ Mucous canal of the head. _mel._ Membrana
elastica externa. _mp._ Muscle-plate. _mp´._ Muscles of muscle-plate. _na._
Neural arch. _nl._ Nervus lateralis. _rp._ Rib process. _sd._ Segmental
duct. _sh._ Sheath of notochord. _spc._ Spinal cord. _spg._ Spinal
ganglion. _syg._ Sympathetic ganglion. _um._ Ductus choledochus. _v._
Blood-vessel. _var._ Vertebral arch. _vb._ Vertebral body. _vcau._ Caudal
vein. _vin._ Intestinal branch of the vagus. _vop._ Ramus ophthalmicus of
the fifth nerve. _x._ Subnotochordal rod.
Fig. 1. Section through the anterior part of an embryo of _Scyllium
canicula_ during stage L.
_c._ Peculiar large cells which are found at the dorsal part of the spinal
cord. Sympathetic ganglion shewn at _syg._ Zeiss A, ocul. 1.
Fig. 2. Section through the lateral line at the time of its first
formation.
The cells marked _nl_ were not sufficiently distinct to make it quite
certain that they really formed part of the lateral nerve. Zeiss B, ocul.
2.
Figs. 3_a_, 3_b_, 3_c_, 3_d_. Four sections of the lateral line from an
embryo belonging to stage L. 3_a_ is the most anterior. In 3_a_ the lateral
nerve (_nl_) is seen to lie in the mesoblast at some little distance from
the lateral line. In 3_b_ and 3_c_ it lies in immediate contact with and
partly enclosed by the modified epiblast cells of the lateral line. In
3_d_, the hindermost section, the lateral line is much larger than in the
other sections, but no trace is present of the lateral nerve. The sections
were taken from the following slides of my series of the embryo (the series
commencing at the tail end) 3_d_ (46), 3_c_ (64), 3_b_ (84), 3_a_ (93). The
figures all drawn on the same scale, but 3 _a_ is not from the same side of
the body as the other sections.
Fig. 4. Section through lateral line of an embryo of stage P at the point
where it is acquiring an opening to the exterior. The peculiar modified
cells of its innermost part deserve to be noticed. Zeiss D, ocul. 2.
Fig. 5. Mucous canals of the head with branches of the ramus ophthalmicus
growing towards them. Stage O. Zeiss A, ocul. 2.
Fig. 6. Mucous canals of head with branches of the ramus ophthalmicus
growing towards them. Stage between O and P. Zeiss a a, ocul. 2.
Fig. 7. Junction of a nerve and mucous canal. Stage P. Zeiss D, ocul. 2.
Fig. 8. Longitudinal and horizontal section through the muscle-plates and
adjoining structures at a stage intermediate between L and M. The section
is intended to shew the gradual conversion of the cells of the somatic
layer of muscle-plates into muscles.
Fig. 9. Longitudinal section through the notochord and adjoining parts to
shew the first appearance of the cartilaginous notochordal sheath which
forms the vertebral centra. Stage N.
Fig. 10. Transverse section through the tail of an embryo of stage P to
shew the coexistence of the rib-process and hæmal arches in the first few
sections behind the point where the latter appear. Zeiss C, ocul. 1.
Fig. 11. Transverse section through the centre of a caudal vertebra of an
embryo somewhat older than Q. It shews (1) the similarity between the
arch-tissue and the hyaline tissue of the outer layer of the vertebral
centrum, and (2) the separation of the two by the membrana elastica
externa[246] (_mel_). It shews also the differentiation of three layers in
the vertebral centrum: vide p. 374.
Footnote 246: The slight difference observable between these
two tissues in the arrangement of their nuclei has been much
exaggerated by the engraver.