THE DEVELOPMENT OF THE ORGANS IN THE HEAD.
_The Development of the Brain._
_General History._ In stage G the brain presents a very simple constitution
(Pl. 8, fig. G), and is in fact little more than a dilated termination to
the cerebro-spinal axis. Its length is nearly one-third that of the whole
body, being proportionately very much greater than in the adult.
It is divided by very slight constrictions into three lobes, the posterior
of which is considerably the largest. These are known as the fore-brain,
the mid-brain, and the hind-brain. The anterior part of the brain is bent
slightly downwards about an axis passing through the mid-brain. The walls
of the brain, composed of several rows of elongated columnar cells, have a
fairly uniform thickness, and even the roof of the hind-brain is as thick
as any other part. Towards the end of stage G the section of the hind-brain
becomes somewhat triangular with the apex of the triangle directed
downwards.
In Pristiurus during stage H no very important changes take place in the
constitution of the brain. In Scyllium, however, indications appear in the
hind-brain of its future division into a cerebellum and medulla oblongata.
The cavity of the anterior part dilates and becomes rounded, while that of
the posterior part assumes in section an hour-glass shape, owing to an
increase in the thickness of the lateral parts of the walls. At the same
time the place of the original thick roof is taken by a very thin layer,
which is formed not so much through a change in the character and
arrangements of the cells composing the roof, as by a divarication of the
two sides of the hind-brain, and the simultaneous introduction of a fresh
structure in the form of a thin sheet of cells connecting dorsally the
diverging lateral halves of this part of the brain. By stage I, the
hind-brain in Pristiurus also acquires an hour-glass shaped section, but
the roof has hardly begun to thin out (Pl. 15, figs. 4_a_ and 4_b_).
During stages I and K the cranial flexure becomes more and more pronounced,
and causes the mid-brain definitely to form the termination of the long
axis of the embryo (Pl. 15, figs. 1, 2, etc.), and before the close of
stage K a thin coating of white matter has appeared on the exterior of the
whole brain, but no other histological changes of interest have occurred.
During stage L an apparent rectification of the cranial flexure commences,
and is completed by stage Q. The changes involved in this process may be
advantageously studied by comparing the longitudinal sections of the brain
during stages L, P, and Q, represented in Pl. 16, figs. 1_a_, 5 and 7_a_.
It will be seen, first of all, that so far from the flexure of the brain
itself being diminished, it is increased, and in P (fig. 5) the angle in
the floor of the mid-brain becomes very acute indeed; in other words, the
anterior part of the brain has been bent upon the posterior through nearly
two right angles, and the infundibulum, or primitive front end of the
brain, now points nearly directly backwards. At the same time the cerebral
hemispheres have grown directly forwards, and if figures 1_a_ and 5 in
Pl. 16 be compared it will be seen that in the older brain of the two the
cerebral hemispheres have assumed a position which might be looked on as
the result of their having been pushed dorsalwards and forwards against the
mid-brain, and having in the process pressed in and nearly obliterated the
original thalamencephalon. The thalamencephalon in fig. 1_a_, belonging to
stage L, is relatively large, but in fig. 5, belonging to stage P, it only
occupies a very small space between the front wall of the mid-brain and the
hind wall of the cerebral hemispheres. It is therefore in part by the
change in position of the cerebral hemispheres that the angle between the
trabeculæ and parachordals becomes increased, _i.e._ their flexure
_diminished_, while at the same time the flexure of the brain itself is
_increased_. More important perhaps in the apparent rectification of the
cranial flexure than any of the previously mentioned points, is the
appearance of a bend in the hind-brain which tends to correct the original
cranial flexure. The gradual growth of this fresh flexure can be studied in
the longitudinal sections which have been represented. It is at its maximum
in stage Q. This short preliminary sketch of the development of the brain
as a whole will serve as an introduction to the history of the individual
divisions of the brain.
_Fore-brain._ In its earliest condition the fore-brain forms a single
vesicle without a trace of separate divisions, but buds off very early the
optic vesicles, whose history is described with that of the eye (Pl. 15,
fig. 3, _op.v_). Between stages I and K the posterior part of the
fore-brain sends outwards a papilliform process towards the exterior, which
forms the rudiment of the pineal gland (Pl. 15, fig. 1, _pn_). Immediately
in front of the rudiment a constriction appears, causing a division of the
fore-brain into a large anterior and a small posterior portion. This
constriction is shallow at first, but towards the close of stage K becomes
much deeper (Pl. 15, fig. 2 and fig. 16_a_), leaving however the two
cavities of the two divisions of the fore-brain united ventrally by a
somewhat wide canal.
The posterior of the two divisions of the fore-brain forms the
thalamencephalon. Its anterior wall adjoining the cerebral rudiment becomes
excessively thin (Pl. 15, fig. 11); and its base till the close of stage K
is in close contact with the mouth involution, and presents but a very
inconspicuous prominence which marks the eventual position of the
infundibulum (Pl. 15, figs. 9_a_, 12, 16_a_, _in_). The anterior and larger
division of the fore-brain forms the rudiment of the cerebral hemispheres
and olfactory lobes. Up to stage K this rudiment remains perfectly simple,
and exhibits no signs, either externally or internally, of a longitudinal
constriction into two lobes. From the canal uniting the two divisions of
the fore-brain (which eventually forms part of the thalamencephalon) there
spring the hollow optic nerves. A slight ventral constriction separating
the cerebral rudiment from that part of the brain where these are attached
appears even before the close of stage K (Pl. 15, fig. 11, _op.n_).
During stage L the infundibulum becomes much produced, and forms a wide
sack in contact with the pituitary body, and its cavity communicates with
that of the third ventricle by an elongated slit-like aperture. This may be
seen by comparing Pl. 16, figs. 1_a_ and 1_c_. In fig. 1_c_ taken along the
middle line, there is present a long opening into the infundibulum (_in_),
which is shewn to be very narrow by being no longer present in fig. 1_a_
representing a section slightly to one side of the middle line. During the
same stage the pineal gland grows into a sack-like body, springing from the
roof of the thalamencephalon, fig. 1_b_, _pn_. This latter (the
thalamencephalon) is now dorsally separated from the cerebral rudiment by a
deep constriction, and also ventrally by a less well marked constriction.
At its side also a deep constriction is being formed in it, immediately
behind the pineal gland. The cerebral rudiment is still quite unpaired and
exhibits no sign of becoming constricted into two lobes.
During the next two stages the changes in the fore-brain are of no great
importance, and I pass at once to stage O. The infundibulum is now nearly
in the same condition as during stage L, though (as is well shewn in the
figure of a longitudinal section of the next stage) it points more directly
backwards than before. The remaining parts of the thalamencephalon have
however undergone considerable changes. The more important of these are
illustrated by a section of stage O, Pl. 16, fig. 3, transverse to the long
axis of the embryo, and therefore, owing to the cranial flexure, cutting
the thalamencephalon longitudinally and horizontally; and for stage P in a
longitudinal and vertical section through the brain (Pl. 16, fig. 5). In
the first place the roof of the thalamencephalon has become very much
shortened by the approximation of the cerebral rudiment to the mid-brain.
The pineal sack has also become greatly elongated, and its somewhat dilated
extremity is situated between the cerebral rudiment and the external skin.
It opens into the hind end of the third ventricle, and its posterior wall
is continuous with the front wall of the mid-brain. The sides of the
thalamencephalon have become much thickened, and form distinct optic
thalami (_op._) united by a very well marked posterior commissure (_pc._).
The anterior wall of the thalamencephalon as well as its roof are very
thin. The optic nerves have become by stage O quite solid except at their
roots, into which the ventricles of the fore-brain are for a short distance
prolonged. This solidification is arrived at, so far as I have determined,
without the intervention of a fold. The nerves are fibrous, and a
commencement of the chiasma is certainly present. From the chiasma there
appears to pass out on each side a band of fibres, which runs near the
outer surface of the brain to the base of the optic lobes (mid-brain), and
here the fibres of the two sides again cross.
By stage O important changes are perceptible in the cerebral rudiment. In
the first place there has appeared a slight fold at its anterior extremity
(Pl. 16, fig. 3, _x_), destined to form a vertical septum dividing it into
two hemispheres, and secondly, lateral outgrowths (vide Pl. 16, fig. 2,
_ol.l_), to form the olfactory lobes. Its thin posterior wall presents on
each side a fold which projects into the central cavity. From the
peripheral end of each olfactory lobe a nerve similar in its histological
constitution to any other cranial nerve makes its appearance (Pl. 16,
fig. 2); this divides into a number of branches, one of which passes into
the connective tissue between the two layers of epithelium in each
Schneiderian fold. On the root of this nerve there is a large development
of ganglionic cells. I have not definitely observed its origin, but have no
reason to doubt that it is a direct outgrowth from the olfactory lobe,
exactly similar _in its mode of development_ to any other nerve of the
body.
The cerebral rudiment undergoes great changes during stage P. In addition
to a great increase in the thickness of its walls, the fold which appeared
in the last stage has grown backwards, and now divides it in front into two
lobes, the rudiments of the cerebral hemispheres. The greater and posterior
section is still however quite undivided, and the cavities of the lobes
(lateral ventricles) though separated in front are still quite continuous
behind. At the same time, the olfactory lobes, each containing a
prolongation of the ventricle, have become much more pronounced (vide
Pl. 16, figs. 4_a_ and 4_c_, _ol.l_). The root of the olfactory nerve is
now very thick, and the ganglion cells it contains are directly prolonged
into the ganglionic portion of the olfactory bulb; in consequence of which
it becomes rather difficult to fix on the exact line of demarcation between
the bulb and the nerve.
Stage Q is the latest period in which I have investigated the development
of the brain. Its structure is represented for this stage in general view
in Pl. 16, figs. 6_a_, 6_b_, 6_c_, in longitudinal section in Pl. 16, figs.
7_a_, 7_b_, and in transverse section Pl. 16, figs. 8_a-d_. The transverse
sections are taken from a somewhat older embryo than the longitudinal. In
the thalamencephalon there is no fresh point of great importance to be
noticed. The pineal gland remains as before, and has become, if anything,
longer than it was, and extends further forwards over the summit of the
cerebrum. It is situated, as might be expected, in the connective tissue
within the cranial cavity (fig. 8_a_, _pn_), and does not extend outside
the skull, as it appears to do, according to Götte's investigations, in
Amphibians. Götte[269] compares the pineal gland with the long persisting
pore which leads into the cavity of the brain in the embryo of Amphioxus,
and we might add the Ascidians, and calls it "ein Umbildungsprodukt einer
letzten Verbindung des Hirns mit der Oberhaut." This suggestion appears to
me a very good one, though no facts have come under my notice which confirm
it. The sacci vasculosi are perhaps indicated at this stage in the two
lateral divisions of the trilobed ventricle of the infundibulum
(fig. 8_c_).
Footnote 269: _Ent. d. Unke_, p. 304.
The lateral ventricles (fig. 8_a_) are now quite separated by a median
partition, and a slight external constriction marks the lobes of the two
hemispheres; these, however, are still united by nervous structures for the
greater part of their extent. The olfactory lobes are formed of a distinct
bulb and stalk (fig. 8_a_, _ol.l_), and contain, as before, prolongations
of the lateral ventricles. The so-called optic chiasma is very distinct
(fig. 8_b_, _op.n_), but the fibres from the optic nerves appear to me
simply to cross and not to intermingle.
_The mid-brain._ The mid-brain is at first fairly marked off from both the
fore and hind brains, but less conspicuously from the latter than from the
former. Its roof becomes progressively thinner and its sides thicker up to
stage P, its cavity remaining quite simple. The thinness of the roof gives
it, in isolated brains of stage P, a bilobed appearance (vide Pl. 16,
fig. 4_b_, _mb_, in which the distinctness of this character is by no means
exaggerated): During stage Q it becomes really bilobed through the
formation in its roof of a shallow median furrow (Pl. 16, fig. 8_b_). Its
cavity exhibits at the same time the indication of a division into a
central and two lateral parts.
_The hind-brain._ The hind-brain has at first a fairly uniform structure,
but by the close of stage I, the anterior part becomes distinguished from
the remainder by the fact, that its roof does not become thin as does that
of the posterior part. This anterior, and _at first very insignificant
portion_, forms the rudiment of the cerebellum. Its cavity is quite simple
and is continued uninterruptedly into that of the remainder of the
hind-brain. The cerebellum assumes in the course of development a greater
and greater prominence, and eventually at the close of stage Q overlaps
both the optic lobes in front and the medulla behind (Pl. 16, fig. 7_a_).
It exhibits in surface-views of the hardened brain of stages P and Q the
appearance of a median constriction, and the portion of the ventricle
contained in it is prolonged into two lateral outgrowths (Pl. 16, figs.
8_c_ and 8_d_, _cb_).
The posterior section of the hind-brain which forms the medulla undergoes
changes of a somewhat complicated character. In the first place its roof
becomes in front very much extended and thinned out. At the raphe, where
the two lateral halves of the brain originally united, a separation, as it
were, takes place, and the two sides of the brain become pushed apart,
remaining united by only a very thin layer of nervous matter (Pl. 15,
fig. 6, _iv.v._). As a result of this peculiar growth in the brain, the
roots of the nerves of the two sides which were originally in contact at
the dorsal summit of the brain become carried away from one another, and
appear to rise at the sides of the brain (Pl. 15, figs. 6 and 7). Other
changes also take place in the walls of the brain. Each lateral wall
presents two projections towards the interior (Pl. 15, fig. 5_a_). The
ventral of these vanish, and the dorsal approximate so as nearly to divide
the cavity of the hind-brain, or fourth ventricle, into a large dorsal and
a small ventral channel (Pl. 15, fig. 6), and this latter becomes
completely obliterated in the later stages. The dorsal pair, while
approximating, also become more prominent, and stretch into the dorsal
moiety of the fourth ventricle (Pl. 15, fig. 6). They are still very
prominent at stage Q (Pl. 16, fig. 8_d_, _ft_), and correspond in position
with the fasciculi teretes of human anatomy. Part of the root of the
seventh nerve originates from them. They project freely in front into the
cavity of the fourth ventricle (Pl. 16, fig. 7_a_, _ft_).
By stage Q restiform tracts are indistinctly marked off from the remainder
of the brain, and are anteriorly continued into the cerebellum, of which
they form the peduncles. Near their junction with the cerebellum they form
prominent bodies (Pl. 16, fig. 7_a_, _rt_), which are regarded by
Miklucho-Maclay[270] as representing the true cerebellum.
Footnote 270: _Das Gehirn d. Selachier_, Leipzig, 1870.
By stage O the medulla presents posteriorly, projecting into its cavity, a
series of lobes which correspond with the main roots (not the branches) of
the vagus and glosso-pharyngeal nerves (Pl. 17, fig. 5). There appear to me
to be present seven or eight projections: their number cannot however be
quite certainly determined. The first of them belongs to the root of the
glosso-pharyngeal, the next one is interposed between the glosso-pharyngeal
and the first root of the vagus, and is without any corresponding
nerve-root. The next five correspond to the five main roots of the vagus.
For each projection to which a nerve pertains there is a special nucleus of
nervous matter, from which the root springs. These nuclei do not stain like
the remainder of the walls of the medulla, and stand out accordingly very
conspicuously in stained sections.
The coating of white matter which appeared at the end of stage K, on the
exterior of each lateral half of the hind-brain, extends from a point just
dorsal to the attachment of the nerve-roots to the ventral edge of the
medulla, and is specially connected with the tissue of the upper of the two
already described projections into the fourth ventricle.
A rudiment of the tela vasculosa makes its appearance during stage Q, and
is represented by the folds in the wall of the fourth ventricle in my
figure of that stage (Pl. 16, fig. 7_a_, _tv_).
* * * * *
The development of the brain in Elasmobranchii has already been worked out
by Professor Huxley, and a brief but in many respects very complete account
of it is given in his recent paper on Ceratodus[271]. He says, pp. 30 and
31, "The development of the cerebral hemispheres in Plagiostome Fishes
differs from the process by which they arise in the higher Vertebrata. In a
very early stage, when the first and second visceral clefts of the embryo
Scyllium are provided with only a few short branchial filaments, the
anterior cerebral vesicle is already distinctly divided into the
thalamencephalon (from which the large infundibulum proceeds below, and the
small tubular peduncle of the pineal gland above, while the optic nerve
leaves its sides) and a large single oval vesicle of the hemispheres. On
the ventral face of the integument covering these are two oval depressions,
the rudimentary olfactory sacs.
Footnote 271: _Proceedings of the Zoological Society_, 1876,
Pt. 1. pp. 30 and 31.
"As development proceeds the vesicle of the hemispheres becomes
divided by the ingrowth of a median longitudinal septum, and the
olfactory lobes grow out from the posterior lateral regions of each
ventricle thus formed, and eventually rise on to the dorsal faces of
the hemispheres, instead of, as in most Vertebrata, remaining on their
ventral sides. I may remark, that I cannot accept the views of
Miklucho-Maclay, whose proposal to alter the nomenclature of the parts
of the Elasmobranch's brain, appears to me to be based upon a
misinterpretation of the facts of development."
The last sentence of the paragraph brings me to the one part on which it is
necessary to say a few words, viz. the views of Miklucho-Maclay. His views
have not received any general acceptance, but the facts narrated in the
preceding pages shew, beyond a doubt, that he has 'misinterpreted' the
facts of development, and that the ordinary view of the homology of the
parts is the correct one. A comparison of the figures I have given of the
embryo brain with similar figures of the brain of higher Vertebrates shews
this point conclusively. Miklucho-Maclay has been misled by the large size
of the cerebellum, but, as we have seen, this body does not begin to be
conspicuous till late in embryonic life. Amongst the features of the
embryonic brain of Elasmobranchii, the long persisting unpaired condition
of the cerebral hemisphere, upon which so much stress has already been laid
by Professor Huxley, appears to me to be one of great importance, and may
not improbably be regarded as a real ancestral feature. Some observations
have recently been published by Professor B. G. Wilder[272] upon this
point, and upon the homologies and development of the olfactory lobes.
Fairly good figures are given to illustrate the development of the cerebral
hemispheres, but the conclusions arrived at are in part opposed to my own
results. Professor Wilder says: "The true hemispheres are the lateral
masses, more or less completely fused in the middle line, and sometimes
developing at the plane of union a bundle of longitudinal commissural
fibres. The hemispheres retain their typical condition as anterior
protrusions of the anterior vesicle; but they lie mesiad of the olfactory
lobes, _and in Mustelus at least seem to be formed after them_." The
italics are my own. From what has been said above, it is clear that the
statement italicised, for Scyllium at least, completely reverses the order
of development. Still more divergent from my conclusions are Professor
Wilder's statements on the olfactory lobes. He says: "The true olfactory
lobe, or rhinencephalon, seems, therefore, to embrace only the hollow base
of the crus, more or less thickened, and more or less distinguishable from
the main mass as a hollow process. The olfactory bulb, with the more or
less elongated crus of many Plagiostomes, seems to be developed
independently, or in connection with the olfactory sack, as are the general
nerves;" and again, "But the young and adult brains since examined shew
that the ventricle (_i.e._ the ventricle of the olfactory lobe) ends as a
rounded cul-de-sac before reaching the 'lobe.'"
Footnote 272: "Anterior brain-mass with Sharks and Skates,"
_American Journal of Science and Arts_, Vol. XII. 1876.
The majority of the statements contained in the above quotations are not
borne out by my observations. Even the few preparations of which I have
given figures, appear to me to prove that (1) the olfactory lobes (crura
and bulbs) are direct outgrowths from the cerebral rudiment, and develop
quite independently of the olfactory sack; (2) that the ventricle of the
cerebral rudiment does not stop short at the base of the crus; (3) that
from the bulb a nerve grows out which has a centrifugal growth like other
nerves of the body, and places the central olfactory lobe in communication
with the peripheral olfactory sack. In some other Vertebrates this nerve
seems hardly to be developed, but it is easily intelligible, that if in the
ordinary course of growth the olfactory sack became approximated to the
olfactory lobe, the nerve which grew out from the latter to the sack might
become so short as to escape detection.
_Organs of Sense._
_The olfactory organ._ The olfactory pit is the latest formed of the three
organs of special sense. It appears during a stage intermediate between _I_
and _K_, as a pair of slight thickenings of the external epiblast, in the
normal vertebrate position on the under side of the fore-brain immediately
in front of the mouth (Pl. 15, figs. 1 and 2, _ol_).
The epiblast cells which form this thickening are very columnar, but
present no special peculiarities. Each thickened patch of skin soon becomes
involuted as a shallow pit, which remains in this condition till the close
of the stage _K_. The epithelium very early becomes raised into a series of
folds (Schneiderian folds). These are bilaterally symmetrical, and diverge
like the barbs of a feather from a median line (Pl. 15, fig. 14). The nasal
pits at the close of stage K are still separated by a considerable interval
from the walls of the brain, and no rudiment of an olfactory lobe arises
till a later period; but a description of the development of this as an
integral part of the brain has already been given, p. 401.
_Eye._ The eye does not present in its early development any very special
features of interest. The optic vesicles arise as hollow outgrowths from
the base of the fore-brain (Pl. 15, fig. 3, _op.v_), from which they soon
become partially constricted, and form vesicles united to the base of the
brain by comparatively narrow hollow stalks, the rudiments of the optic
nerves. The constriction to which the stalk or optic nerve is due takes
place from above and backwards, so that the optic nerves open into the base
of the front part of the thalamencephalon (Pl. 15, fig. 13_a_, _op.n_).
After the establishment of the optic nerves, there take place the formation
of the lens and the pushing in of the anterior wall of the optic vesicle
towards the posterior.
The lens arises in the usual vertebrate fashion. The epiblast in front of
the optic vesicle becomes very much thickened, and then involuted as a
shallow pit, which eventually deepens and narrows. The walls of the pit are
soon constricted off as a nearly spherical mass of cells enclosing a very
small central cavity, in some cases indeed so small as to be barely
recognizable (Pl. 15, fig. 7, _l_). The pushing in of the anterior wall of
the optic vesicle towards the posterior takes place in quite the normal
manner; but, as has been already noticed by Götte[273] and others, is not a
simple mechanical result of the formation of the lens, as is shewn by the
fact that the vesicle assumes a flattened form even before the appearance
of the lens. The whole exterior of the optic cup becomes invested by
mesoblast, but _no mesoblastic cells grow in between the lens and the
adjoining wall of the optic cup_.
Footnote 273: _Entwicklungsgeschichte d. Unke._
Round the exterior of the lens, and around the exterior and interior of the
optic cup, there appear membrane-like structures, similar to those already
described round the spinal cord and other organs. These membrane-like
structures appear with a varying distinctness, but at the close of stage
_K_ stand out with such remarkable clearness as to leave no doubt that they
are not artificial products (Pl. 15, fig. 13_a_)[274]. They form the
rudiments of the hyaloid membrane and lens capsule. Similar, though less
well marked membranes, may often be seen lining the central cavity of the
lens and the space between the two walls of the optic cup. The optic cup is
at first very shallow, but owing to the rapid growth of the free edge of
its walls soon becomes fairly deep. The growth extends to the whole
circumference of the walls except the point of entrance of the optic nerve
(Pl. 15, fig. 13_a_), where no growth takes place; here accordingly a gap
is left in the walls which forms the well-known choroid slit. While this
double walled cup is increasing in size, the wall lining the cavity of the
cup becomes thick, and the outer wall very thin (fig. 13_a_). No further
differentiations arise before the close of stage K.
Footnote 274: The engraver has not been very successful in
rendering these membranes.
The lens is carried outwards with the growth of the optic cup, leaving the
cavity of the cup quite empty. It also grows in size, and its central
cavity becomes larger. Still later its anterior wall becomes very thin, and
its posterior wall thick, and doubly convex (fig. 13_a_). Its changes,
however, so exactly correspond to those already known in other Vertebrates,
that a detailed description of them would be superfluous.
_No mesoblast passes into the optic cup round its edge_, but a process of
mesoblast, accompanied by a blood-vessel, passes into the space between the
lens and the wall of the optic cup through the choroid slit (fig. 13_a_,
_ch_). This process of tissue is very easily seen, and swells out on
entering the optic cup into a mushroom-like expansion. It forms the
processus falciformis, and from it is derived the vitreous humour.
About the development of the parts of the eye, subsequently to stage _K_, I
shall not say much. The iris appears during stage O, as an ingrowing fold
of both layers of the optic cup with a layer of mesoblast on its outer
surface, which tends to close over the front of the lens. Both the epiblast
layers comprising the iris are somewhat atrophied, and the outer one is
strongly pigmented. At stage O the mesoblast first also grows in between
the external skin and the lens to form the rudiment of the mesoblastic
structures of the eye in front of the lens. The layer, when first formed,
is of a great tenuity.
The points in my observations, to which I attach the greatest importance,
are the formation of the lens capsule and the hyaloid membrane; with the
development of these may be treated also that of the vitreous humour and
rudimentary _processus falciformis_. The development of these parts in
Elasmobranchii has recently been dealt with by Dr Bergmeister[275], and his
observations with reference to the vitreous humour and processus
falciformis, the discovery of which in embryo Elasmobranchii is due to him,
are very complete. I cannot, however, accept his view that the hyaloid
membrane is a mesoblastic product. Through the choroid slit there grows, as
has been said, a process of mesoblast, the processus falciformis, which on
entering the optic cup dilates, and therefore appears mushroom-shaped in
section. At the earliest stage (K) a blood-vessel appeared in connection
with it, but no vascular structure came under my notice in the later
stages. The structure of this process during stage P is shewn in Pl. 17,
fig. 6, _p.fal._; it is there seen to be composed of mesoblast-cells with
fibrous prolongations. The cells, as has been noticed by Bergmeister, form
a special border round its dilated extremity. This process is formed much
earlier than the vitreous humour, which is first seen in stage O. In
hardened specimens this latter appears either as a gelatinous mass with a
meshwork of fibres or (as shewn in Pl. 17, fig. 6) with elongated fibres
proceeding from the end of the processus falciformis. These fibres are
probably a product of the hardening reagent, but perhaps represent some
preformed structure in the vitreous humour. I have failed to detect in it
any cellular elements. It is more or less firmly attached to the hyaloid
membrane.
Footnote 275: "Embryologie d. Coloboms," _Sitz. d. k. Akad.
Wien_, Bd. LXXI. 1875.
On each side of the processus falciformis in stage P a slight fold of the
optic cup is to be seen, but folds so large as those represented by
Bergmeister have never come under my notice, though this may be due to my
not having cut sections of such late embryos as he has. The hyaloid
membrane appears long before the vitreous humour as a delicate basement
membrane round the inner surface of the optic cup (Pl. 15, fig. 13_a_),
which is perfectly continuous with a similar membrane round the outer
surface. In the course of development the hyaloid membrane becomes thicker
than the membrane outside the optic cup, with which however it remains
continuous. This is very clear in my sections of stage M. By stage O the
membrane outside the cup has ceased to be distinguishable, but the hyaloid
membrane may nevertheless be traced to the very edge of the cup round the
developing iris; but does not unite with the lens capsule. It can also be
traced quite to the junction of the two layers of the optic cup at the side
of the choroid slit (Pl. 17, fig. 6, _hy.m_). When the vitreous humour
becomes artificially separated from the retina, the hyaloid membrane
sometimes remains attached to the former, but at other times retains in
preference its attachment to the retina. My observations do not throw any
light upon the junction of the hyaloid membrane and lens capsule to form
the suspensory ligament, nor have I ever seen (as described by Bergmeister)
the hyaloid membrane extending across the free end of the processus
falciformis and separating the latter from the vitreous humour. This
however probably appears at a period subsequent to the latest one
investigated by me. The lens capsule arises at about the same period as the
hyaloid membrane, and is a product of the cells of the lens. It can be very
distinctly seen in all the stages subsequent to its first formation. The
proof of its being a product of the epiblastic lens, and not of the
mesoblast, lies mainly in the fact of there being no mesoblast at hand to
give rise to it at the time of its formation, vide Pl. 15, fig. 13_a_. If
the above observations are correct, it is clear that the hyaloid membrane
and lens capsule are respectively products of the retina and lens; so that
it becomes necessary to go back to the older views of Kölliker and others
in preference to the more modern ones of Lieberkühn and Arnold. It would
take me too far from my subject to discuss the arguments used by the later
investigators to maintain their view that the hyaloid membrane and lens
capsule are mesoblastic products; but it will suffice to say that the
continuity of the hyaloid membrane over the pecten in birds is no
conclusive argument against its retinal origin, considering the great
amount of apparently independent growth which membranes, when once formed,
are capable of exhibiting.
Bergmeister's and my own observations on the vitreous humour clearly prove
that this is derived from an ingrowth through the choroid slit. On the
other hand, the researches of Lieberkühn and Arnold on the Mammalian Eye
appear to demonstrate that a layer of mesoblast becomes in Mammalia
involuted with the lens, and from this the vitreous humour (including the
_membrana capsulo-pupillaris_) is said to be in part formed. Lieberkühn
states that in Birds the vitreous humour is formed in a similar fashion. I
cannot, however, accept his results on this point. It appears, therefore,
that, so far as is known, all groups of Vertebrata, with the exception of
Mammalia, conform to the Elasmobranch type. The differences between the
types of Mammalia and remaining Vertebrata are, however, not so great as
might at first sight appear. They are merely dependent on slight
differences in the manner in which the mesoblast enters the optic cup. In
the one case it grows in round one specialized part of the edge of the cup,
_i.e._ the choroid slit; in the other, round the whole edge, including the
choroid slit. Perhaps the mode of formation of the vitreous humour in
Mammalia may be correlated with the early closing of the choroid slit.
_Auditory Organ._ With reference to the development of the organ of hearing
I have very little to say. Opposite the interval between the seventh and
the glosso-pharyngeal nerves the external epiblast becomes thickened, and
eventually involuted as a vesicle which remains however in communication
with the exterior by a narrow duct. Towards the close of stage K the
auditory sack presents three protuberances--one pointing forwards, a second
backwards, and a third outwards. These are respectively the rudiments of
the anterior and posterior vertical and external horizontal semicircular
canals. These rudiments are easily visible from the exterior (Pl. 15,
fig. 2).
* * * * *
As has been already pointed out, the epiblast of Elasmobranchii during the
early periods of development exhibits no division into an epidermic and a
nervous layer, and in accordance with its primitive undifferentiated
condition, those portions of the organs of sense which are at this time
directly derived from the external integument are formed indiscriminately
from the whole, and not from an inner or so-called nervous part of it only.
In the Amphibians the auditory sack and lens are derived from the nervous
division of the epiblast only, while the same division of the layer plays
the major part in forming the olfactory organ. It is also stated that in
Birds and Mammals the part of the epiblast corresponding to the nervous
layer is alone concerned in the formation of the lens, though this does not
appear to be the case with the olfactory or auditory organs in these groups
of Vertebrates.
_Mouth involution and Pituitary body._
The development of the mouth involution and the pituitary body is closely
related to that of the brain, and may conveniently be dealt with here. The
epiblast in the angle formed by the cranial flexure becomes involuted as a
hollow process situated in close proximity to the base of the brain. This
hollow process is the mouth involution, and it is bordered on its posterior
surface by the front wall of the alimentary tract, and on its anterior by
the base of the fore-brain.
The uppermost end of this does not till near the close of stage K become
markedly constricted off from the remainder, but is nevertheless the
rudiment of the pituitary body. Pl. 15, figs. 9_a_ and 12, _m_ shew in a
most conclusive manner the correctness of the above account, and
demonstrate that it is from the mouth involution, and not, as has usually
been stated, from the alimentary canal, that the pituitary body is derived.
This fact was mentioned in my preliminary account of Elasmobranch
development[276]; and has also been shewn to be the case in Amphibians by
Götte[277]; and in Birds by Mihalkowics[278]. The fact is of considerable
importance with reference to speculations as to the meaning of this body.
Footnote 276: _Quarterly Journal of Microscopic Science_, Oct.
1874.
Footnote 277: _Entwicklungsgeschichte der Unke._ Götte was the
first to draw attention to this fact. His observations were
then shewn to hold true for Elasmobranchii by myself, and
subsequently for Birds by Mihalkowics.
Footnote 278: _Arch. f. micr. Anat._ Vol. XI.
Plate 15, fig. 7 represents a transverse section through the head during a
stage between I and K; but, owing to the cranial flexure, it cuts the fore
part of the head longitudinally and horizontally, and passes through both
the fore-brain (_fb_) and the hind-brain (_iv.v._). Close to the base of
the fore-brain are seen the mouth (_m_), and the pituitary involution from
this (_pt._). In contact with the pituitary involution is the blind
anterior termination of the throat, which a little way back opens to the
exterior by the first visceral cleft (I. _v.c._). This figure alone
suffices to demonstrate the correctness of the above account of the
pituitary body; but the truth of this is still further confirmed by other
figures on the same plate (figs. 9_a_ and 12, _m_); in which the mouth
involution is in contact with, but still separated from, the front end of
the alimentary tract. By the close of stage K, the septum between the mouth
and throat becomes pierced, and the two are placed in communication. This
condition is shewn in Pl. 15, fig. 16_a_, and Pl. 16, figs. 1_a_, 1_c_,
_pt_ In these figures the pituitary involution has become very partially
constricted off from the mouth involution, though still in direct
communication with it. In later stages the pituitary involution becomes
longer and dilated terminally, while the passage connecting it with the
mouth becomes narrower and narrower, and is finally reduced to a solid
cord, which in its turn disappears. The remaining vesicle then becomes
divided into lobes, and connects itself closely with the infundibulum (Pl.
16, figs. 5 and 6 _pt_). The later stages for Elasmobranchii are fully
described by W. Müller in his important memoir on the Comparative Anatomy
and development of this organ[279].
Footnote 279: W. Müller, "Ueber Entwicklung and Bau d.
Hypophysis u. d. Processus infundibuli cerebri," _Jenaische
Zeitschrift_, Bd. VI.
_Development of the Cranial Nerves._
The present section deals with the whole development (so far as I have
succeeded in elucidating it) of the cranial nerves (excluding the optic and
olfactory nerves and the nerves of the eye-muscles) from their first
appearance to their attainment of the adult condition. My description
commences with the first development of the nerves, to this succeeds a
short description of the nerves in the adult Scyllium, and the section is
completed by an account of the gradual steps by which the adult condition
is attained.
_Early Development of the Cranial Nerves._--Before the close of stage H the
more important of the cranial nerves make their appearance. The fifth and
the seventh are the first to be formed. The fifth arises by stage G
(Pl. 15, fig. 3, V), near the anterior end of the hind-brain, as _an
outgrowth from the extreme dorsal summit of the brain, in identically the
same way as the dorsal root of a spinal nerve_.
The roots of the two sides sprout out from the summit of the brain, in
contact with each other, and grow ventralwards, one on each side of the
brain, in close contact with its walls. I have failed to detect more than
one root for the two embryonic branches of the fifth (ophthalmic and
mandibular), _and no trace of an anterior or ventral root has been met with
in any of my sections_.
The seventh nerve is formed nearly simultaneously with or shortly after the
fifth, and some little distance behind and independently of it, opposite
the anterior end of the thickening of the epiblast to form the auditory
involution. It arises precisely like the fifth, from the extreme dorsal
summit of the neural axis (Pl. 15, fig. 4_a_, VII). So far as I have been
able to determine, the auditory nerve and the seventh proper possess only a
single root common to the two. There is no anterior root for the seventh
any more than for the fifth.
Behind the auditory involution, at a stage subsequent to that in which the
fifth and seventh nerves appear, there arise a series of roots from the
dorsal summit of the hind-brain, which form the rudiments of the
glosso-pharyngeal and vagus nerves. These roots are formed towards the
close of stage H, but are still quite short at the beginning of stage I.
Their manner of development resembles that of the previously described
cranial nerves. The central ends of the roots of the opposite sides are at
first in contact with each other, and there is nothing to distinguish the
roots of the glosso-pharyngeal and of the vagus nerves from the dorsal
roots of spinal nerves. Like the dorsal roots of the spinal nerves, they
appear as a series of ventral prolongations of a continuous outgrowth from
the brain, which outgrowth is moreover continuous with that for the spinal
nerves[280]. The outgrowth of the vagus and glosso-pharyngeal nerves is not
continuous with that of the seventh nerve. This is shewn by Pl. 15, figs.
4_a_ and 4_b_. The outgrowth of the seventh nerve though present in 4_a_ is
completely absent in 4_b_ which represents a section just behind 4_a_.
Footnote 280: In the presence of this continuous outgrowth of
the brain from which spring the separate nerve stems of the
vagus, may perhaps be found a reconciliation of the apparently
conflicting statements of Götte and myself with reference to
the vagus nerve. Götte regards the vagus as a single nerve,
from its originating as an undivided rudiment; but it is clear
from my researches that, for Elasmobranchii at least, this
method of arguing will not hold good, since it would lead to
the conclusion that all the spinal nerves were branches of one
single nerve, since they too spring as processes from a
continuous outgrowth from the brain!
Thus, by the end of stage I, there have appeared the rudiments of the 5th,
7th, 8th, 9th and 10th cranial nerves, all of which spring from the
hind-brain. These nerves all develop precisely as do the posterior roots of
the spinal nerves, and it is a remarkable fact _that hitherto I have failed
to find a trace in the brain of a root of any cranial nerve arising from
the ventral corner of the brain as do the anterior roots of the spinal
nerves_[281].
Footnote 281: The conclusion here arrived at with reference to
the anterior roots, is opposed to the observations of both
Gegenbaur on Hexanchus, _Jenaische Zeitschrift_, Vol. VI., and
of Jackson and Clarke on Echinorhinus, _Journal of Anatomy and
Physiology_, Vol. X. These morphologists identify certain roots
springing from the medulla below and behind the main roots of
the vagus as true anterior roots of this nerve. The existence
of these roots is not open to question, but without asserting
that it is impossible for me to have failed to detect such
roots had they been present in the embryo, I think I may
maintain if these anterior roots are not present in the embryo,
their identification as vagus roots must be abandoned; and they
must be regarded as belonging to spinal nerves. This point is
more fully spoken of at p. 428.
It is admittedly difficult to prove a negative, and it may still turn out
that there are anterior roots of the brain similar to those of the spinal
cord; in the mean time, however, the balance of evidence is in favour of
there being none such. This at first sight appears a somewhat startling
conclusion, but a little consideration shews that it is not seriously
opposed to the facts which we know. In the first place it has been shewn by
myself[282] that in Amphioxus (whose vertebrate nature I cannot doubt) only
dorsal nerve-roots are present. Yet the nerves of Amphioxus are clearly
mixed motor and sensory nerves, and it appears to me far more probable that
Amphioxus represents a phase of development in which the nerves had not
acquired two roots, rather than one in which the anterior root has been
lost. In other words, the condition of the nerves in Amphioxus appears to
me to point to the conclusion _that primitively the cranio-spinal nerves of
vertebrates were nerves of mixed function with one root only, and that root
a dorsal one; and that the present anterior or ventral root is a secondary
acquisition_. This conclusion is further supported by the fact that the
posterior roots develop in point of time before the anterior roots. If it
be admitted that the vertebrate nerves primitively had only a single root,
then the retention of that condition in the brain implies that this became
differentiated from the remainder of the nervous system at a very early
period before the acquirement of anterior nerve-roots, and that these
eventually become developed only in the case of spinal nerves, and not in
the case of the already highly modified cranial nerves.
Footnote 282: _Journal of Anatomy and Physiology_, Vol. X.
[This Edition, No. IX.]
* * * * *
_Subsequent Changes of the Nerves._ To simplify my description of the
subsequent growth of the cranial nerves, I have inserted a short
description of their distribution in the adult. This is taken from a
dissection of Scyllium stellare, which like other species has some
individualities of its own not found in the other Elasmobranchii. For
points not touched on in this description I must refer the reader to the
more detailed accounts of my predecessors, amongst whom may specially be
mentioned Stannius[283] for Carcharias, Spinax, Raja, Chimæra, &c.;
Gegenbaur[284] for Hexanchus; Jackson and Clarke[285] for Echinorhinus.
Footnote 283: _Nervensystem d. Fische_, Rostock, 1849.
Footnote 284: _Jenaische Zeitschrift_, Vol. VI.
Footnote 285: _Journal of Anatomy and Physiology_, Vol. X.
The ordinary nomenclature has been employed for the branches of the fifth
and seventh nerves, though embryological data to be adduced in the sequel
throw serious doubts upon it. Since I am without observations on the origin
of the nerves to the muscles of the eyes, all account of these is omitted.
The fifth nerve arises from the brain by three roots[286]: (1) an
anterior more or less ventral root; (2) a root slightly behind, but
close to the former[287], formed by the coalescence of two distinct
strands, one arising from a dorsal part of the medulla, and a second
and larger from the ventral; (3) a dorsal and posterior root, in its
origin quite distinct and well separated from the other two, and
situated slightly behind the dorsal strand of the second root. This
root a little way from its attachment becomes enclosed for a short
distance in the same sheath as the dorsal part of the second root, and
a slight mixture of fibres seems to occur, but the majority of its
fibres have no connection with those of the second root. The first and
second roots of the fifth appear to me partially to unite, but before
their junction the ramus ophthalmicus profundus is given off from the
first of them.
Footnote 286: My results with reference to these roots accord
exactly, so far as they go, with the more carefully worked out
conclusions of Stannius, _loc. cit._ pp. 29 and 30.
Footnote 287: The root of the seventh nerve cannot properly be
distinguished from this root.
The fifth nerve, according to the usual nomenclature, has three main
divisions. The first of these is the ophthalmic. It is formed by the
coalescence of two entirely independent branches of the fifth, which
unite on leaving the orbit. The dorsalmost of these, or ramus
ophthalmicus superficialis, originates from the third and posterior of
the roots of the fifth, nearly the whole of which appears to enter
into its formation. This root is situated on the dorsal part of the
"lobi trigemini," _at a point posterior to that of the other roots of
the fifth or even of the seventh nerve_. The branch itself enters the
orbit by a separate foramen, and, keeping on the dorsal side of it,
reenters the cartilage at its anterior wall, and is there joined by
the _ramus ophthalmicus profundus_. This latter nerve arises from the
anterior root of the fifth, separately pierces the wall of the orbit,
and takes a course slightly ventral to the superior ophthalmic nerve,
but does not (as is usual with Elasmobranchii) run below the superior
rectus and superior oblique muscles of the eye. The nerve formed by
the coalescence of the superficial and deep ophthalmic branches
courses a short way below the surface, and supplies the mucous canals
of the front of the snout. It is a purely sensory nerve. Strong
grounds will be adduced in the sequel for regarding the _ramus
ophthalmicus superficialis_, though not the _ophthalmicus profundus_,
as in reality a branch of the seventh, and not of the fifth nerve.
The second division of the fifth nerve is the superior maxillary,
which appears to me to arise from both the first and second roots of
the fifth, though mainly from the first. It divides once into two main
branches. The first of these--the buccal nerve of Stannius--after
passing forwards along the base of the orbit takes its course
obliquely across the palatine arch and behind and below the nasal
sack, supplying by the way numerous mucous canals, and dividing at
last into two branches, one of these passing directly forwards on the
ventral surface of the snout, and the second keeping along the front
border of the mouth. The second division of the superior maxillary
nerve (superior maxillary of Stannius), after giving off a small
branch, which passes backwards in company with a branch from the
inferior maxillary nerve to the levator maxillæ superioris, itself
keeps close to the buccal nerve, and eventually divides into numerous
fine twigs to the mucous canals of the skin at the posterior region of
the upper jaw. It anastomoses with the buccal nerve. The inferior
maxillary nerve arises mainly from the second root of the fifth. After
sending a small branch to the levator maxillæ superioris, it passes
outwards along the line separating the musculus adductor mandibulæ
from the musculus levator labii superioris, and after giving branches
to these muscles takes a course forward along the border of the lower
jaw. It appears to be a mixed motor and sensory nerve.
The seventh or facial nerve arises by a root close to, but behind and
below the second root of the fifth, and is intimately fused with this.
It divides almost at once into a small anterior branch and large
posterior.
The anterior branch is the palatine nerve. It gives off at first one
or two very small twigs, which pursue a course towards the spiracle,
and probably represent the spiracular nerves of other Elasmobranchii.
Immediately after giving off these branches it divides into two stems,
a posterior smaller and an anterior larger one. The former eventually
takes a course which tends towards the angle of the jaw, and is
distributed to the mucous membrane of the roof of the mouth, while the
larger one bends forwards and supplies the mucous membrane at the edge
of the upper jaw. The main stem of the seventh, after giving off a
branch to the dorsal section of the musculus constrictor
superficialis, passes outwards to the junction of the upper and lower
jaws, where it divides into two branches, an anterior superficial
branch, which runs immediately below the skin on the surface of the
lower jaw, and a second branch, which takes a deep course along the
posterior border of the lower jaw, between it and the hyoid, and sends
a series of branches backwards to the ventral section of the musculus
constrictor superficialis. The main stem of the facial is mixed motor
and sensory. I have not noticed a dorsal branch, similar to that
described by Jackson and Clarke.
The auditory nerve arises immediately behind the seventh, but requires
no special notice here. A short way behind the auditory is situated
the root of the glossopharyngeal nerve. This nerve takes an oblique
course backwards through the skull, and gives off in its passage a
very small dorsal branch, which passes upwards and backwards through
the cartilage towards the roof of the skull. At the point where the
main stem leaves the cartilage it divides into two branches, an
anterior smaller branch to the hinder border of the hyoid arch, and a
posterior and larger one to anterior border of the first branchial
arch. It forks, in fact, over the first visceral cleft.
The vagus arises by a great number of distinct strands from the sides
of the medulla. In the example dissected there were twelve in all. The
anterior three of these were the largest; the middle one having the
most ventral origin. The next four were very small and in pairs, and
were separated by a considerable interval from the next four, also
very small, and these again by a marked interval from the hindermost
strand.
The common stem formed by the junction of these gives off immediately
on leaving the skull a branch which forks on the second branchial
cleft; a second for the third cleft is next given off; the main stem
then divides into a dorsal branch--the lateral nerve--and a ventral
one--the branchio-intestinal nerve--which, after giving off the
branches for the two last branchial clefts, supplies the heart and
intestinal tract. The lateral nerve passes back towards the posterior
end of the body, internal to the lateral line, and between the
dorso-lateral and ventro-lateral muscles. It gives off at its origin a
fine nerve, which has a course nearly parallel to its own. The main
stem of the vagus, at a short distance from its central end, receives
a nerve which springs from the ventral side of the medulla, on about a
level with the most posterior of the true roots of the vagus. This
small nerve corresponds with the ventral or anterior roots of the
vagus described by Gegenbaur, Jackson, and Clarke (though in the
species investigated by the latter authors these roots did not join
the vagus, but the anterior spinal nerves). Similar roots are also
mentioned by Stannius, who found two of them in the Elasmobranchii
dissected by him; it is possible that a second may be present in
Scyllium, but have been overlooked by me, or perhaps may have been
exceptionally absent in the example dissected.
_The Fifth Nerve._ The thinning of the roof of the brain, in the manner
already described, produces a great change in the apparent position of the
roots of all the nerves. The central ends of the rudiments of the two sides
are, as has been mentioned, at first in contact dorsally but, when by the
growth of the roof of the brain its two lateral halves become pushed apart,
the nerves also shift their position and become widely separated. The roots
of the fifth nerve are so influenced by these changes that they spring from
the brain about half way up its sides, and a little ventral to the border
of its thin roof. While this change has been taking place in the point of
attachment of the fifth nerve, it has not remained in other respects in a
stationary condition.
During stage H it already exhibits two distinct branches known as the
mandibular and ophthalmic. These branches first lie outside a section of
the body-cavity which exists in the front part of the head. The ophthalmic
branch of the fifth being situated near the anterior end of this, and the
mandibular near the posterior end.
In stage I the body-cavity in this part becomes divided into two parts one
behind the other, the posterior being situated in the mandibular arch. The
bifurcation of the nerve then takes place over the summit of the posterior
of the two divisions of the body-cavity, Pl. 15, figs. 9_b_, V, and 10, V,
&c., and at first both branches keep close to the sides of this.
The anterior or ophthalmic branch of the fifth soon leaves the walls of the
cavity just spoken of and tends towards the eye, and there comes in close
contact with the most anterior section of the body-cavity which exists in
the head. These relations it retains unchanged till the close of stage K.
Between stages I and K it may easily be seen from the surface; but, before
the close of stage K, the increased density of the tissues renders it
invisible in the living embryo.
The posterior branch of the fifth extends downwards into the mandibular
arch in close contact with the posterior and outer wall of the body space
already alluded to. At first no branches from it can be seen, but I have
detected by the close of stage K, by an examination of the living embryo, a
branch springing from it a short way from its central extremity, and
passing forwards, Pl. 15, fig. 2, V This branch I take to be the rudiment
of the superior maxillary division of the fifth nerve. It is shewn in
section, Pl. 15, fig. 15_a_, V.
In the stages after K the anatomy of the nerves becomes increasingly
difficult to follow, and accordingly I must plead indulgence for the
imperfections in my observations on all the nerves subsequently to this
date. In the fifth I find up to stage O a single ophthalmic branch (Pl. 17,
fig. 4_b_, V._op.th._), which passes forwards slightly dorsal to the eye
and parallel and ventral to a branch of the seventh, which will be
described when I come to that nerve. I have been _unable_ to observe that
this branch divides into a ramus superficialis and ramus profundus, and
subsequently to stage O I have no observations on it.
By stage O the fifth may be observed to have two very distinct roots, and a
large ganglionic mass is developed close to their junction (Gasserian
ganglion), Pl. 17, fig. 4_a_. But in addition to this ganglionic
enlargement, all of the branches have special ganglia of their own, Pl. 17,
fig. 4_b_.
_Summary._ The fifth nerve has almost from the beginning two branches, the
ophthalmic (probably the inferior ophthalmic of the adult) and the inferior
maxillary. The superior maxillary nerve arises later than the other two as
a branch from the inferior, originating comparatively far from its root.
There is at first but a single root for the whole nerve, which subsequently
becomes divided into two. Ganglionic swellings are developed on the common
stem and main branches of the nerve.
A general view of the nerve is shewn in the diagram in Pl. 17, fig. 1.
* * * * *
_Seventh and Auditory Nerves._ There appears in my earliest sections a
single large rudiment in the position of the seventh and auditory nerves;
but in longitudinal sections of an embryo somewhat older than stage I, in
which the auditory organ forms a fairly deep pit, still widely open to the
exterior, there are to be seen immediately in front of the ear the
rudiments of two nerves, which come into contact where they join the brain
and have their roots still closely connected at the end of stage K (Pl. 15,
figs. 10 and 15_a_ and 15_b_). The anterior of these pursues a straight
course to the hyoid arch (Pl. 15, fig. 10, VII), the second of the two
(Pl. 15, fig. 10, _au.n._), which is clearly the rudiment of the auditory
nerve, develops a ganglionic enlargement and, turning backward, closely
hugs the ventral wall of the auditory involution.
The observation just recorded appears to lead to the following conclusions
with reference to the development of the auditory nerve. A single rudiment
arises from the brain for the auditory and seventh nerves. This rudiment
subsequently becomes split into two parts, an anterior to form the seventh
nerve, and a posterior to form the auditory nerve. The ganglionic part of
the auditory nerve is derived from the primitive outgrowths from the brain,
and not from the auditory involution. I do not feel perfectly confident
that an independent origin of the auditory nerve might not have escaped my
notice; but, admitting the correctness of the view which attributes to the
seventh and auditory a common origin, it follows that the auditory nerve
primitively arose in connection with the seventh, of which it may either,
as Gegenbaur believes, be a distinct part--the ramus dorsalis--or else may
possibly have formed part of a commissure, homologous with that uniting the
dorsal roots of the spinal nerves, connecting the seventh with the
glossopharyngeal nerve. In either case it must be supposed secondarily to
have become separate and independent in consequence of the development of
the organ of hearing.
My sections of embryos of stage K and the subsequent stages do not bring to
light many new facts with reference to the auditory nerve: they demonstrate
however that its ganglionic part increases greatly in size, and in stage O
there is a distinct root for the auditory nerve in contact with that for
the seventh.
The history of the seventh nerve in its later stages presents points of
great interest. Near the close of stage K there may be observed, in the
living embryos and in sections, two branches of the seventh in addition to
the original trunk to the hyoid arch, both arising from its anterior side;
one passes straight forwards close to the external skin, but is at first
only traceable a short way in front of the fifth, and a second passes
downwards into the mandibular arch in such a fashion, that the seventh
nerve forks over the hyomandibular cleft (vide Pl. 15, fig. 2, VII.; 15_a_,
VII.). My sections shew both these branches with great clearness. A third
branch has also come under my notice, whose course leads me to suppose that
it supplies the roof of the palate.
In the later stages my attention has been specially directed to the very
remarkable anterior branch of the seventh. This may, in stages L to O, be
traced passing on a level with the root of the fifth nerve above the eye,
and apparently terminating in branches to the skin in front of the eye
(Pl. 17, figs. 3, VII.; 4_a_, VII,_a_). It courses close beneath the skin
(though this does not appear in the sections represented on account of
their obliqueness), and runs parallel and dorsal to the ophthalmic branch
of the fifth nerve, and may easily be seen in this position in longitudinal
sections belonging to stage O; but its changes after this stage have
hitherto baffled me, and its final fate is therefore, to a certain extent,
a matter of speculation.
The two other branches of the seventh, viz., the hyoid or main branch and
mandibular branch, retain their primitive arrangement till the close of
stage O.
The fate of the remarkable anterior branch of the seventh nerve is one of
the most interesting points which has started up in the course of my
investigations on the development of the cranial nerves, and it is a matter
of very great regret to me that I have not been able to clear up for
certain its later history.
Its primitive distribution leads to the supposition that it becomes the
nerve known in the adult as the _ramus ophthalmicus superficialis of the
fifth nerve_, and this is the view which I admit myself to be inclined to
adopt. There are several points in the anatomy of this nerve in the adult
which tell in favour of accepting this view with reference to it. In the
first place, the ramus ophthalmicus superficialis rises from the brain
(vide description above, p. 417), quite independently of the ramus
ophthalmicus profundus, and not in very close connection with the other
branches of the fifth, and also considerably behind these, quite as far
back indeed as the ventral root of the seventh. There is therefore nothing
in the position of its root opposed to its being regarded as a branch of
the seventh nerve. Secondly, its distribution, which might at first sight
be regarded as peculiar, presents no very strange features if it is looked
on as a ramus dorsalis of the seventh, whose apparent anterior instead of
dorsal course is due to the cranial flexure. If, however, the distribution
of the ramus ophthalmicus superficialis is used as an argument against my
view, a satisfactory reply is to be found in the fact that a branch of the
seventh nerve certainly has the distribution in question _in the embryo_,
and that there is no reason why it should not retain it _in the adult_.
Finally, the junction of the two rami ophthalmici, most remarkable if they
are branches of a single nerve, would present nothing astonishing when they
are regarded as branches of two separate nerves.
If this view be adopted, certain modifications of the more generally
accepted views of the morphology of the cranial nerves will be
necessitated; but this subject is treated of at the end of this section.
Some doubt hangs over the fate of the other branches of the seventh nerve,
but their destination is not so obscure as that of the anterior branch. The
branch to the roof of the mouth can be at once identified as the 'palatine
nerve', and it only remains to speak of the mandibular branch.
It may be noticed first of all with reference to this branch, that the
seventh behaves precisely like the less modified succeeding cranial nerves.
It forks in fact over a visceral cleft (the hyomandibular) the two sides of
which it supplies; the branch at the anterior side of the cleft is the
later developed and smaller of the two. There cannot be much doubt that the
mandibular branch must be identified with the spiracular nerve
(præ-spiracular branch Jackson and Clarke) of the adult, and if the chorda
tympani of Mammals is correctly regarded as the mandibular branch of the
seventh nerve, then the spiracular nerve must represent it. Jackson and
Clarke[288] take a different view of the homology of the chorda tympani,
and regard it as equivalent to the ramus mandibularis internus (one of the
two branches into which the seventh eventually divides), because this nerve
takes its course over the ligament connecting the mandible with the hyoid.
This view I cannot accept so long as it is admitted that the chorda tympani
is the branch of a cranial nerve supplying the anterior side of a cleft.
The ramus mandibularis internus, instead of forming with the main branch of
the seventh a fork over the spiracle, passes to its destination completely
behind and below the spiracle, and therefore fails to fulfil the conditions
requisite for regarding it as a branch to the anterior wall of a visceral
cleft. It is indeed clear that the ramus mandibularis internus cannot be
identified with the embryonic mandibular branch of the seventh (which
passes above the spiracle or hyomandibular cleft) when there is present in
the adult another nerve (the spiracular nerve), which exactly corresponds
in distribution with the embryonic nerve in question. My view accords
precisely with that already expressed by Gegenbaur in his masterly paper on
the nerves of Hexanchus, in which he distinctly states that he looks upon
the spiracular nerve as the homologue of an anterior branchial branch of a
division of the vagus. In the adult the spiracular nerve is sometimes
represented by one or two branches of the palatine, _e.g._ Scyllium, but at
other times arises independently from the main stem of the seventh[289].
The only difficulty in my identification of the embryonic mandibular branch
with the adult spiracular nerve, is the extremely small size of the latter
in the adult, compared with the size of mandibular in the embryo; but it is
hardly surprising to find an atrophy of the spiracular nerve accompanying
an atrophy of the spiracle itself. The palatine appears to me to have been
rightly regarded by Jackson and Clarke as the great superficial petrosal of
Mammals.
Footnote 288: _Loc. cit._
Footnote 289: Hexanchus, Gegenbaur, _Jenaische Zeitschrift_, Vol. VI.
On the common root of the branches of the seventh nerve, as well as on its
hyoid branch, ganglionic enlargements are present at an early period of
development.
_The Glossopharyngeal and Vagus Nerves._ Behind the ear there are formed a
series of five nerves which pass down to respectively the first, second,
third, fourth and fifth visceral arches.
For each arch there is thus one nerve, whose course lies close to the
posterior margin of the preceding cleft, a second anterior branch being
developed later. These nerves are connected with the brain (as I have
determined by transverse sections) by roots at first attached to the dorsal
summit, but eventually situated about half-way down the sides (Pl. 15,
fig. 6) nearly opposite the level of the process which divides the
ventricle of the hind-brain into a dorsal and a ventral moiety. The
foremost of these nerves is the glossopharyngeal. The next four are, as has
been shewn by Gegenbaur[290], equivalent to four independent nerves, but
form, together with the glossopharyngeal, a compound nerve, which we may
briefly call the vagus.
Footnote 290: _Loc. cit._
This compound nerve by stage K attains a very complicated structure, and
presents several remarkable and unexpected features. Since it has not been
possible for me completely to elucidate the origin of all its various
parts, it will conduce to clearness if I give an account of its structure
during stage K or L, and then return to what facts I can mention with
reference to its development. Its structure during these stages is
represented on the diagram, Pl. 17, fig. 1. There are present five
branches, viz. the glossopharyngeal and four branches of the vagus, arising
probably by a considerably greater number of strands from the brain[291].
All the strands from the brain are united together by a thin commissure,
_Vg.com._, continuous with the commissure of the posterior roots of the
spinal nerves, and from this commissure the five branches are continued
obliquely ventralwards and backwards, and each of them dilates into a
ganglionic swelling. They all become again united together by a second
thick commissure, which is continued backwards as the intestinal branch of
the vagus nerve _Vg.in._ The nerves, however, are continued ventralwards
each to its respective arch. From the hinder part of the intestinal nerve
springs the lateral nerve _n.l._, at a point whose relations to the
branches of the vagus I have not certainly determined.
Footnote 291: In the diagram there are only five strands
represented. This is due to the fact that I have not certainly
made out their true number.
The whole nerve-complex formed by the glossopharyngeal and the vagus nerves
cannot of course be shewn in any single section. The various roots are
shewn in Pl. 17, fig. 5. The dorsal commissure is represented in
longitudinal section in Pl. 15, fig. 15_b_, _com._, and in transverse
section in Pl. 17, fig. 2, _Vg.com_ The lower commissure continued as the
intestinal nerve is shewn in Pl. 15, fig. 15_a_, _Vg._, and as seen in the
living embryo in Pl. 15, figs. 1 and 2. The ganglia are seen in Pl. 15,
fig. 6, _Vg_. The junction of the vagus and glossopharyngeal nerves is
shewn in Pl. 15, fig. 10. My observations have not taught me much with
reference to the origin of the two commissures, viz. the dorsal one and the
one which forms the intestinal branch of the vagus. Very possibly they
originate as a single commissure which becomes longitudinally segmented. It
deserves to be noticed that the dorsal commissure has a long stretch, from
the last branch of the vagus to the first spinal nerve, during which it is
not connected with the root of any nerve; vide fig. 15_b_, _com_. This
space probably contained originally the now lost branches of the vagus. In
many transverse sections where the dorsal commissure might certainly be
expected to be present it cannot be seen, but this is perhaps due to its
easily falling out of the sections. I have not been able to prove that the
commissure is continued forwards into the auditory nerve.
The relation of the branches of the vagus and glossopharyngeal to the
branchial clefts requires no special remark. It is fundamentally the same
in the embryo as in the adult. The branches at the posterior side of the
clefts are the first to appear, those at the anterior side of the clefts
being formed subsequently to stage K.
One of the most interesting points with reference to the vagus is the
number of separate strands from the brain which unite to form it. The
questions connected with these have been worked out in a masterly manner,
both from an anatomical and a theoretical standpoint, by Professor
Gegenbaur[292]. It has not been possible for me to determine the exact
number of these in my embryos, nor have I been able to shew whether they
are as numerous at the earliest appearance of the vagus as at a later
embryonic period. The strands are connected (Pl. 17, fig. 5) with separate
ganglionic centres in the brain, though in several instances more than one
strand is connected with a single centre. In an embryo between stage O and
P more than a dozen strands are present. In an adult Scyllium I counted
twelve separate strands, but their number has been shewn by Gegenbaur to be
very variable. It is possible that they are remnants of the roots of the
numerous primary branches of the vagus which have now vanished; and this
perhaps is the explanation of their variability, since in the case of all
organs which are on the way to disappear variability is a precursor of
disappearance.
Footnote 292: _Loc. cit._
A second interesting point is the presence of the two connecting
commissures spoken of above. It was not till comparatively late in my
investigations that I detected the dorsal one. This has clearly the same
characters as the dorsal commissure already described as connecting the
roots of all the spinal nerves, and is indeed a direct prolongation of
this. It becomes gradually thinner and thinner, and finally ceases to be
observable by about the close of stage L. It is of importance as shewing
the similarity of the branches of the vagus to the dorsal roots of the
spinal nerves. The ventral of the two commissures persists in the adult as
the common stem from which all the branches of the vagus successively
originate, and is itself continued backwards as the intestinal branch of
the vagus. The glossopharyngeal nerve alone becomes eventually separated
from the succeeding branches. Stannius and Gegenbaur have, as was mentioned
above, detected in adult Elasmobranchii roots which join the vagus, and
which resemble the anterior or ventral roots of spinal nerves; and I have
myself described one such root in the adult Scyllium. I have searched for
these in my embryos, but without obtaining conclusive results. In the
earliest stages I can find no trace of them, but I have detected in stage L
one anterior root on debatable border-land, which may conceivably be the
root in question, but which I should naturally have put down for the root
of a spinal nerve. Are the roots in question to be regarded as proper roots
of the vagus, or as ventral roots of spinal nerves whose dorsal roots have
been lost? The latter view appears to me the most probable one, partly from
the embryological evidence furnished by my researches, which is clearly
opposed to the existence of anterior roots in the brain, and partly from
the condition of these roots in Echinorhinus, in which they join the
succeeding spinal nerves and not the vagus[293]. The similar relations of
the apparently homologous branch or branches in many Osseous Fish may also
be used as an argument for my view.
Footnote 293: Vide Jackson and Clarke, _loc. cit._ The authors
take a different view to that here advocated, and regard the
ventral roots described by them as having originally belonged
to the vagus.
If, as seems probable, the roots in question become the hypoglossal nerve,
this nerve must be regarded as formed from the anterior roots of one or
more spinal nerves. Without embryological evidence it does not however seem
possible to decide whether the hypoglossal nerve contains elements only of
anterior roots or of both anterior and posterior roots.
_Mesoblast of the Head._
_Body-Cavity and Myotomes of the Head._--During stage F the appearance of a
cavity on each side in the mesoblast of the head was described. (Vide
Pl. 10, figs. 3_b_ and 6, _pp_.) These cavities end in front opposite the
blind anterior extremity of the alimentary canal; behind they are
continuous with the general body-cavity. I propose calling them the
_head-cavities_. The cavities of the two sides have no communication with
each other.
Coincidently with the formation of an outgrowth from the throat to form the
first visceral cleft, the head-cavity on each side becomes divided into a
section in front of the cleft and a section behind the cleft (vide Pl. 15,
figs. 4_a_ and 4_b_, _pp._); and during stage H it becomes, owing to the
formation of a second cleft, divided into three sections: (1) a section in
front of the first or hyomandibular cleft; (2) a section in the hyoid arch
between the hyomandibular cleft and the hyobranchial or first branchial
cleft; (3) a section behind the first branchial cleft.
The section in front of the hyomandibular cleft stands in a peculiar
relation to the two branches of the fifth nerve. The ophthalmic branch of
the fifth lies close to the outer side of its anterior part, the mandibular
branch close to the outer side of its posterior part. During stage I this
front section of the head-cavity grows forward, and becomes divided,
without the intervention of a visceral cleft, into an anterior and
posterior division. The anterior lies close to the eye, and in front of the
commencing mouth involution, and is connected with the ophthalmic branch of
the fifth nerve. The posterior part lies completely within the mandibular
arch, and is closely connected with the mandibular division of the fifth
nerve.
As the rudiments of the successive visceral clefts are formed, the
posterior part of the head-cavity becomes divided into successive sections,
there being one section for each arch. Thus the whole head-cavity becomes
on each side divided into (1) a premandibular section; (2) a mandibular
section; (3) a hyoid section; (4) sections in the branchial arches.
The first of these divisions forms a space of a considerable size, with
epithelial walls of somewhat short columnar cells. It is situated close to
the eye, and presents a rounded or sometimes triangular figure in sections
(Pl. 15, figs. 7, 9_b_ and 16_b_, 1_pp._). The ophthalmic branch of the
fifth nerve passes close to its superior and outer wall.
Between stages I and K the anterior cavities of the two sides are prolonged
ventralwards and meet below the base of the fore-brain (Pl. 15, fig. 8,
1_pp._). The connection between the two cavities appears to last for a
considerable time, and still persists at the close of stage L. The anterior
or premandibular pair of cavities are the only parts of the body-cavity
within the head which unite ventrally. In the trunk, however, the
primitively independent lateral halves of the body-cavity always unite in
this way. The section of the head-cavity just described is so similar to
the remaining posterior sections that it must be considered as equivalent
to them.
The next division of the head-cavity, which from its position may be called
the mandibular cavity, presents during the stages I and K a spatulate
shape. It forms a flattened cavity, dilated dorsally, and produced
ventrally into a long thin process parallel to the hyomandibular
gill-cleft, Pl. 15, fig. 1_pp._ and fig. 7, 9_b_ and 15_a_, 2_pp_. Like the
previous space it is lined by a short columnar epithelium.
The fifth nerve, as has already been mentioned, bifurcates over its dorsal
summit, and the mandibular branch of that nerve passes down on its
posterior and outer side. The mandibular aortic arch is situated close to
its inner side, Pl. 15, fig. 7. Towards the close of this period the upper
part of the cavity atrophies. Its lower part also becomes much narrowed,
but its walls of columnar cells persist and lie close to one another. The
outer or somatic wall becomes very thin indeed, the splanchnic wall, on the
other hand, thickens and forms a layer of several rows of elongated cells.
This thicker wall is on its inner side separated from the surrounding
tissue by a small space lined by a membrane-like structure. In each of the
remaining arches there is a segment of the original body-cavity
fundamentally similar to that in the mandibular arch. A dorsal dilated
portion appears, however, to be present in the third or hyoid section
alone, and even there disappears by the close of stage K. The cavities in
the posterior parts of the head become much reduced like those in its
anterior part, though at rather a later period. Their walls however
persist, and become more columnar. In Pl. 15, fig. 13_b_, _pp._, is
represented the cavity in the last arch but one, at a period when the
cavity in the mandibular arch has become greatly reduced. It occupies the
same position on the outer side of the aortic trunk of its arch as does the
cavity in the mandibular arch (Pl. 15, fig. 7, 2 _pp_). In Torpedo embryos
the head-cavity is much smaller, and atrophies earlier than in the embryos
of Pristiurus and Scyllium.
It has been shewn that, with the exception of the most anterior, the
divisions of the body-cavity in the head become atrophied, _not so however
their walls_. The cells forming these become elongated, and by stage N
become distinctly developed into muscles. Their exact history I have not
followed in its details, but they almost unquestionably become the musculus
constrictor superficialis and musculus interbranchialis[294]; and probably
also musculus levator mandibuli and other muscles of the front part of the
head.
Footnote 294: Vide Vetter, "Die Kiemen und Kiefermusculatur d.
Fische." _Jenaische Zeitschrift_, Vol. VII.
The most anterior cavity close to the eye remains unaltered much longer
than the remaining cavities, and its two halves are still in communication
at the close of stage L. I have not yet succeeded in tracing the subsequent
fate of its walls, _but think it probable that they develop into the
muscles of the eye_. The morphological importance of the sections of the
body-cavity in the head cannot be over-estimated, and the fact that the
walls become developed into the muscular system of the head renders it
almost certain _that we must regard them as equivalent to the muscle-plates
of the body, which originally contain, equally with those of the head,
sections of the body-cavity_. If this determination is correct, there can
be no doubt that they ought to serve as valuable guides to the number of
segments which have coalesced to form the head. This point is, however,
discussed in a subsequent section.
_General mesoblast of the head._--In stage G no mesoblast is present in the
head, except that which forms the walls of the head-cavity.
During stage H a few cells of undifferentiated connective tissue appear
around the stalk of the optic vesicle, and in the space between the front
end of the alimentary tract and the base of the brain in the angle of the
cranial flexure. They are probably budded off from the walls of the
head-cavities. Their number rapidly increases, and they soon form an
investment surrounding all the organs of the head, and arrange themselves
as a layer, between the walls of the roof of the fore and mid-brain and the
external skin. At the close of stage K they are still undifferentiated and
embryonic, each consisting of a large nucleus surrounded by a very delicate
layer of protoplasm produced into numerous thread-like processes. They form
a regular meshwork, the spaces of which are filled up by an intercellular
fluid.
I have not worked out the development of the cranial and visceral skeleton;
but this has been made the subject of an investigation by Mr Parker, who is
more competent to deal with it than any other living anatomist. His results
were in part made known in his lectures before the Royal College of
Surgeons[295], and will be published in full in the _Transactions of the
Zoological Society_.
Footnote 295: A report of the lectures appeared in _Nature_.
All my efforts have hitherto failed to demonstrate any segmentation in the
mesoblast of the head, other than that indicated by the sections of the
body-cavity before-mentioned; but since these, as above stated, must be
regarded as equivalent to muscle-plates, any further segmentation of
mesoblast could not be anticipated. To this statement the posterior part of
the head forms an apparent exception. Not far behind the auditory
involution there are visible at the end of period K a few longitudinal
muscles, forming about three or four muscle-plates, the ventral part of
which is wanting. I have not the means of deciding whether they properly
belong to the head, or may not really be a part of the trunk system of
muscles which has, to a certain extent, overlapped the back part of the
head, but am inclined to accept the latter view. These cranial
muscle-plates are shewn in Pl. 15, fig. 15_b_, and in Pl. 17, fig. 2.
_Notochord in the Head._
The notochord during stage G is situated for its whole length close under
the brain, and terminates opposite the base of the mid-brain. As the
cranial flexure becomes greater and mesoblast is collected in the angle
formed by this, the termination of the notochord recedes from the base of
the brain, but remains in close contact with the front end of the
alimentary canal. At the same time its terminal part becomes very much
thinner than the remainder, ends in a point, and exhibits signs of a
retrogressive metamorphosis. It also becomes bent upon itself in a ventral
direction through an angle of 180°; vide Pl. 15, figs. 9_a_ and 16_a_. In
some cases this curvature is even more marked than is represented in these
figures.
The bending of the end of the notochord is not directly caused by the
cranial flexure, as is proved by the fact that the end of the notochord
becomes bent through a far greater angle than does the brain. During the
stages subsequent to K the ventral flexure of the notochord disappears, and
its terminal part acquires by stage O a distinct dorsal curvature.
_Hypoblast of the Head._
The only feature of the alimentary tract in the head which presents any
special interest is the formation of the gill-slits and of the thyroid
body. In the present section the development of the former alone is dealt
with; the latter body will be treated in the section devoted to the general
development of the alimentary tract.
The gill-slits arise as outgrowths of the lining of the throat towards the
external skin. In the gill-slits of Torpedo I have observed a very slight
ingrowth of the external skin towards the hypoblastic outgrowth in one
single case. In all other cases observed by me, the outgrowth from the
throat meets the passive external skin, coalesces with it, and then, by the
dissolution of the wall separating the lumen of the throat from the
exterior, a free communication from the throat outwards is effected; vide
Pl. 15, figs. 5_a_ and _b_, and 13_b_. Thus it happens that the walls
lining the clefts are entirely formed of hypoblast. The clefts are formed
successively[296], the anterior appearing first, and it is not till after
the rudiments of three have appeared, that any of them become open to the
exterior.
Footnote 296: Vide Plate 8.
In stage K, four if not five are open to the exterior, and the rudiments of
six, the full number, have appeared[297]. Towards the close of stage K
there arise, from the walls of the 2nd, 3rd and 4th clefts, very small
knob-like processes, the rudiments of the external gills. These outgrowths
are formed both by the lining of the gill-cleft and by the adjoining
mesoblast[298].
Footnote 297: The description of stage K and L, pp. 292 and
293, is a little inaccurate with reference to the number of the
visceral clefts, though the number visible in the hardened
embryos is correctly described.
Footnote 298: Vide on the development of the gills, Schenk,
_Sitz. d. k. Akad. Wien_, Vol. LXXI, 1875.
From the mode of development of the gill-clefts, it appears that their
walls are lined externally by hypoblast, and therefore that the external
gills are processes of the walls of the alimentary tract, _i.e._ are
covered by an hypoblastic, and not an epiblastic layer. It should be
remembered, however, that after the gill-slits become open, the point where
the hypoblast joins the epiblast ceases to be determinable, so that some
doubt hangs over the above statement.
The identification of the layer to which the gills belong is not without
interest. If the external gills have an epiblastic origin, they may be
reasonably regarded[299] as homologous with the external gills of Annelids;
but, if derived from the hypoblast, this view becomes, to say the least,
very much less probable.
Footnote 299: Vide Dohrn, _Ursprung d. Wirbelthiere_.
_Segmentation of the Head._
The nature of the vertebrate head and its relation to the trunk forms some
of the oldest questions of Philosophical Morphology.
The answers of the older anatomists to these questions are of a
contradictory character, but within the last few years it has been more or
less generally accepted that the head is, in part at least, merely a
modified portion of the trunk, and composed, like that, of a series of
homodynamous segments[300]. While the researches of Huxley, Parker,
Gegenbaur, Götte, and other anatomists, have demonstrated in an
approximately conclusive manner that the head is composed of a series of
segments, great divergence of opinion still exists both as to the number of
these segments, and as to the modifications which they have undergone,
especially in the anterior part of the head. The questions involved are
amongst the most difficult in the whole range of morphology, and the
investigations recorded in the preceding pages do not, I am very well
aware, go far towards definitely solving them. At the same time my
observations on the nerves and on the head-cavities appear to me to throw a
somewhat new light upon these questions, and it has therefore appeared to
me worth while shortly to state the results to which a consideration of
these organs points. There are three sets of organs, whose development has
been worked out, each of which presents more or less markedly a segmental
arrangement:--(1) The cranial nerves; (2) the visceral clefts; (3) the
divisions of the head-cavity.
Footnote 300: Semper, in his most recent work, maintains, if I
understand him rightly, that the head is in no sense a modified
part of the trunk, but admits that it is segmented in a similar
fashion to the trunk.
The first and second of these have often been employed in the solution of
the present problem, while the third, so far as is known, exists only in
the embryos of Elasmobranchii.
The development of the cranial nerves has recently been studied with great
care by Dr Götte, and his investigations have led him to adopt very
definite views on the segments of head. The arrangement of the cranial
nerves _in the adult_ has frequently been used in morphological
investigations about the skull, but there are to my mind strong grounds
against regarding it as affording a safe basis for speculation. The most
important of these depends on the fact that nerves are liable to the
greatest modification on any changes taking place in the organs they
supply. On this account it is a matter of great difficulty, amounting in
many cases to actual impossibility, to determine the morphological
significance of the different nerve-branches, or the nature of the fusions
and separations which have taken place at the roots of the nerves. It is,
in fact, only in those parts of the head which have, relatively speaking,
undergone but slight modifications, and which require no special
elucidation from the nerves, that these sufficiently retain in the adult
their primitive form to serve as trustworthy morphological guides.
I propose to examine separately the light thrown on the segmentation of the
head by the development of (1) the nerves, (2) the visceral clefts, (3) the
head-cavities; and then to compare the three sets of results so obtained.
The post-auditory nerves present no difficulties; they are all organized in
the same fashion, and, as was first pointed out by Gegenbaur, form five
separate nerves, each indicating a segment. A comparison of the
post-auditory nerves of Scyllium and other typical Elasmobranchii with
those of Hexanchus and Heptanchus proves, however, that other segments were
originally present behind those now found in the more typical forms. And
the presence in Scyllium of numerous (twelve) strands from the brain to
form the vagus, as well as the fact that a large section of the commissure
connecting the vagus roots with the posterior roots of the spinal nerves is
not connected with the brain, appear to me to shew that all traces of the
lost nerves have not yet vanished.
Passing forwards from the post-auditory nerves, we come to the seventh and
auditory nerves. The embryological evidence brought forward in this paper
is against regarding these nerves as representing two segments. Although it
must be granted that my evidence is not conclusive against an independent
formation of these two nerves, yet it certainly tells in favour of their
originating from a common rudiment, and Marshall's results on the origin of
the two nerves in Birds (published in the _Journal of Anatomy and
Physiology_, Vol. XI. Part 3) support, I have reason to believe, the same
conclusion. Even were it eventually to be proved that the auditory nerve
originated independently of the seventh, the general relations of this
nerve, embryological and otherwise, are such that, provisionally at least,
it could not be regarded as belonging to the same category as the facial or
glossopharyngeal nerves, and it has therefore no place in a discussion on
the segmentation of the head.
The seventh nerve of the embryo (Pl. 17, fig. 1, VII) is formed by the
junction of three conspicuous branches, (1) an anterior dorsal branch which
takes a more or less horizontal course above the eye (VII. _a_); (2) a main
branch to the hyoid arch (VII. _hy_); (3) a smaller branch to the posterior
edge of the mandibular arch (VII. _mn_). The first of these branches can
clearly be nothing else but the typical "ramus dorsalis," of which however
the auditory may perhaps be a specialized part. The fact that this branch
pursues an anterior and not a directly dorsal course is probably to be
explained as a consequence of the cranial flexure. The two other branches
of the seventh nerve are the same as those present in all the posterior
nerves, viz. the branches to the two sides of a branchial cleft, in the
present instance the spiracle; the seventh nerve being clearly the nerve of
the hyoid arch.
The fifth nerve presents in the arrangement of its branches a similarity to
the seventh nerve so striking that it cannot be overlooked. This similarity
is at once obvious from an inspection of the diagram of the nerves on
Pl. 17, fig. 1, V, or from an examination of the sections representing
these nerves (Pl. 17, figs. 3 and 4). It divides like the seventh nerve
into three main branches: (1) an anterior and dorsal branch (_r._
ophthalmicus profundus), whose course lies parallel to but ventral to that
of the dorsal branch of the seventh nerve; (2) a main branch to the
mandibular arch (_r._ maxillæ inferioris); and (3) an anterior branch to
the palatine arcade (_r._ maxillæ superioris). I was at first inclined to
regard the anterior branch of the fifth (ophthalmic) as representing a
separate nerve, and was supported in this view by its relation to the most
anterior of the head-cavities; but the unexpected discovery of an exactly
_similar branch_ in the seventh nerve has induced me to modify this view,
and I am now constrained to view the fifth as a single nerve, whose
branches exactly correspond with those of the seventh. The anterior branch
of the fifth is, like the corresponding branch of the seventh, the _ramus
dorsalis_, and the two other branches are the equivalent of the branches of
the seventh, which fork over the spiracle, though in the case of the fifth
nerve no distinct cleft is present unless we regard the mouth as such.
Embryology thus appears to teach us that the fifth nerve is a single nerve
supplying the mandibular arch, and not, as has been usually thought, a
complex nerve resulting from the coalescence of two or three distinct
nerves. My observations do not embrace the origin or history of the third,
fourth, and sixth nerves, but it is hardly possible to help suspecting that
in these we have the nerve of one or more segments in front of that
supplied by the fifth nerve; a view which well accords with the most recent
morphological speculations of Professor Huxley[301].
Footnote 301: Preliminary note upon the brain and skull of
Amphioxus, _Proc. of the Royal Society_, Vol. XXII.
From this enumeration of the nerves the optic nerve is excluded for obvious
reasons, and although it has been shewn above that the olfactory nerve
develops like the other nerves as an outgrowth from the brain, yet its very
late appearance and peculiar relations are, at least for the present, to my
mind sufficient grounds for excluding it from the category of segmental
cranial nerves.
The nerves then give us indications of seven cranial segments, or, if the
nerves to the eye-muscles be included, of _at the least_ eight segments,
but to these must be added a number of segments now lost, but which once
existed behind the last of those at present remaining.
The branchial clefts have been regarded as guides to segmentation by
Gegenbaur, Huxley, Semper, etc., and this view cannot I think be
controverted. In Scyllium there are six clefts which give indications of
seven segments, viz., the segments of the mandibular arch, hyoid arch, and
of the five branchial arches. If, following the views of Dr Dohrn[302], we
regard the mouth as representing a cleft, we shall have seven clefts and
eight segments; and it is possible, as pointed out in Dr Dohrn's very
suggestive pamphlet, that remnants of a still greater number of præoral
clefts may still be in existence. Whatever may be the value of these
speculations, such forms as Hexanchus and Heptanchus and Amphioxus make it
all but certain that the ancestors of Vertebrates had a number of clefts
behind those now developed.
Footnote 302: _Ursprung d. Wirbelthiere._
The last group of organs to be dealt with for our present question is that
of the Head-Cavities.
The walls of the spaces formed by the cephalic prolongations of the
body-cavity develop into muscles and resemble the muscle-plates of the
trunk, and with these they must be identified, as has been already stated.
As equivalent to the muscle-plates, they clearly are capable of serving as
very valuable guides for determining the segmentation of the head. There
are then a pair of these in front of the mandibular arch, a pair in the
mandibular arch, and a pair in each succeeding arch. In all there are eight
pairs of these cavities representing eight segments, the first of them
præoral. As was mentioned above, each of the sections of the head-cavity
(except perhaps the first) stands in a definite relation to the nerve and
artery of the arch in which it is situated.
The comparative results of these three independent methods of determining
the segmentation of the head are in the subjoined table represented in a
form in which they can be compared:--
_Table of the Cephalic Segments as determined by the Nerves, Visceral
Arches, and Head-Cavities._
+----------+---------------------+------------------+----------------+
| Segments | Nerves | Visceral Arches | Head-Cavities |
| | | | or Cranial |
| | | | Muscle-Plates |
|----------+---------------------+------------------+----------------+
|Præoral 1 |3rd and 4th and | (?) |1st head cavity |
| |? 6th nerves (perhaps| |(in my figures |
| |representing more | | 1_pp._) |
| |than one segment) | | |
| | | | |
|Postoral 2|5th nerve |Mandibular |2nd head-cavity |
| | | |(in my figures |
| | | | 2_pp._) |
| | | | |
| ---- 3|7th nerve |Hyoid |3rd head-cavity |
| | | | |
| ---- 4|Glossopharyngeal |1st branchial arch|4th head-cavity |
| |nerve | | |
| | | | |
| ---- 5|1st branch of vagus |2nd branchial arch|5th head-cavity |
| | | | |
| ---- 6|2nd branch of vagus |3rd branchial arch|6th head-cavity |
| | | | |
| ---- 7|3rd branch of vagus |4th branchial arch|7th head-cavity |
| | | | |
| ---- 8|4th branch of vagus |5th branchial arch|8th head-cavity |
+----------+---------------------+------------------+----------------+
In the above table the first column denotes the segments of the head as
indicated by a comparison of the three sets of organs employed. The second
column denotes the segments as obtained by an examination of the nerves;
the third column is for the visceral arches (which lead to the same results
as, but are more convenient for our table than, the visceral clefts), and
the fourth column is for the head-cavities. It may be noticed that from the
second segment backwards the three sets of organs lead to the same results.
The head-cavities indicate one segment in front of the mouth, and now that
the ophthalmic branch of the fifth has been dethroned from its position as
a separate nerve, the eye-nerves, or one of them, may probably be regarded
as belonging to this segment. If the suggestion made above (p. 431), that
the walls of the first cavity become the eye-muscles, be correct, the
eye-nerves would perhaps after all be the most suitable nerves to regard as
belonging to the segment of the first head-cavity.
EXPLANATION OF PLATES 15, 16, 17.
PLATE 15. (THE HEAD DURING STAGES G--K.)
COMPLETE LIST OF REFERENCE LETTERS.
1_aa_, 2_aa_, etc. 1st, 2d, etc. aortic arch. _acv._ Anterior cardinal
vein. _al._ Alimentary canal. _ao._ Aorta. _au._ Thickening of epiblast to
form the auditory pit. _aun._ Auditory nerve. _aup._ Auditory pit. _auv._
Auditory vesicle. _b._ Wall of brain. _bb._ Base of brain. _cb._
Cerebellum. _cer._ Cerebrum. _Ch._ Choroid slit. _ch._ Notochord. _com._
Commissure connecting roots of vagus nerve. 1, 2, 3 etc. _eg._ External
gills. _ep._ External epiblast. _fb._ Fore-brain. _gl._ Glossopharyngeal
nerve. _hb._ Hind-brain. _ht._ Heart. _hy._ Hyaloid membrane. _In._
Infundibulum. _l._ Lens. _M._ Mouth involution. _m._ Mesoblast at the base
of the brain. _mb._ Mid-brain. _mn._ v. Mandibular branch of fifth. _ol._
Olfactory pit. _op._ Eye. _opn._ Optic nerve. _opv._ Optic vesicle.
_opth_V. Ophthalmic branch of fifth. _p._ Posterior root of spinal nerve.
_pn._ Pineal gland. 1, 2 etc. _pp._ First, second, etc. section of
body-cavity in the head. _pt._ Pituitary body. _so._ Somatopleure. _sp._
Splanchnopleure. _spc._ Spinal cord. _Th._ Thyroid body. _v._ Blood-vessel.
iv._v._ Fourth ventricle. v. Fifth nerve. _Vc._ Visceral cleft. _Vg._
Vagus. vii. Seventh or facial nerve.
Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent
object.
The points which deserve special attention are: (1) The sections of the
body-cavity in the head (_pp_): the first or premandibular section being
situated close to the eye, the second in the mandibular arch. Above this
one the fifth nerve bifurcates. The third at the summit of the hyoid arch.
The cranial nerves and the general appearance of the brain are well shewn
in the figure.
The notochord cannot be traced in the living embryo so far forward as it is
represented. It has been inserted according to the position which it is
seen to occupy in sections.
Fig. 2. Head of an embryo of Scyllium canicula somewhat later than stage K,
viewed as a transparent object.
The figure shews the condition of the brain; the branches of the fifth and
seventh nerves (v. vii.); the rudiments of the semicircular canals; and the
commencing appearance of the external gills as buds on both walls of 2nd,
3rd, and 4th clefts. The external gills have not appeared on the first
cleft or spiracle.
Fig. 3. Section through the head of a Pristiurus embryo during stage G. It
shews (1) the fifth nerve (v.) arising as an outgrowth from the dorsal
summit of the brain. (2) The optic vesicles not yet constricted off from
the fore-brain.
Figs. 4_a_ and 4_b_. Two sections through the head of a Pristiurus embryo
of stage I. They shew (1) the appearance of the seventh nerve. (2) The
portion of the body-cavity belonging to the first and second visceral
arches. (3) The commencing thickening of epiblast to form the auditory
involution.
In 4_b_, the posterior of the two sections, no trace of an auditory nerve
is to be seen.
Figs. 5_a_ and 5_b_. Two sections through the head of a Torpedo embryo with
3 visceral clefts. Zeiss A, ocul. 1.
5_a_ shews the formation of the thin roof of the fourth ventricle by a
divarication of the two lateral halves of the brain.
Both sections shew the commencing formation of the thyroid body (_th_) at
the base of the mandibular arch.
They also illustrate the formation of the visceral clefts by an outgrowth
from the alimentary tract without any corresponding ingrowth of the
external epiblast.
Fig. 6. Section through the hind-brain of a somewhat older Torpedo embryo.
Zeiss A, ocul. 1.
The section shews (1) the attachment of a branch of the vagus to the walls
of the hind-brain. (2) The peculiar form of the hind-brain.
Fig. 7. Transverse section through the head of a Pristiurus embryo
belonging to a stage intermediate between I and K, passing through both the
fore-brain and the hind-brain. Zeiss A, ocul. 1.
The section illustrates (1) the formation of the pituitary body (_pt_) from
the mouth involution (_m_), and proves that, although the wall of the
throat (_al_) is in contact with the mouth involution, there is by this
stage no communication between the two. (2) The eye. (3) The sections of
the body-cavity in the head (1_pp_, 2_pp_). (4) The fifth nerve (v.) and
the seventh nerve (vii).
Fig. 8. Transverse section through the brain of a rather older embryo than
fig. 7. It shews the ventral junction of the anterior sections of the
body-cavity in the head (1_pp_).
Figs. 9_a_ and 9_b_. Two longitudinal sections through the brain of a
Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss
A, ocul. 1.
9_a_ is taken through the median line, but is reconstructed from two
sections. It shews (1) The divisions of the brain--The cerebrum and
thalamencephalon in the fore-brain; the mid-brain; the commencing
cerebellum in the hind-brain. (2) The relation of the mouth involution to
the infundibulum. (3) The termination of the notochord.
9_b_ is a section to one side of the same brain. It shews (1) The divisions
of the brain. (2) The point of outgrowth of the optic nerves (_opn_). (3)
The sections of the body-cavity in the head and the bifurcation of the
optic nerve over the second of these.
Fig. 10. Longitudinal section through the head of a Pristiurus embryo
somewhat younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of
the nerves and the junction of the fifth, seventh, and auditory nerves with
the brain.
Fig. 11. Longitudinal section through the fore-brain of a Pristiurus embryo
of stage K, slightly to one side of the middle line. It shews the deep
constriction separating the thalamencephalon from the cerebral hemispheres.
Fig. 12. Longitudinal section through the base of the brain of an embryo of
a stage intermediate between I and K.
It shews (1) the condition of the end of the notochord; (2) the relation of
the mouth involution to the infundibulum.
Fig. 13_a_. Longitudinal and horizontal section through part of the head of
a Pristiurus embryo rather older than K. Zeiss A, ocul. 1.
The figure contains the eye cut through in the plane of the choroid slit.
Thus the optic nerve (_opn_) and choroid slit (_ch_) are both exhibited.
Through the latter is seen passing mesoblast accompanied by a blood-vessel
(_v_). _Op_ represents part of the optic vesicle to one side of the choroid
slit.
No mesoblast can be seen passing round the outside of the optic cup; and
the only mesoblast which enters the optic cup passes through the choroid
slit.
Fig. 13_b_. Transverse section through the last arch but one of the same
embryo as 13_a_. Zeiss A, ocul. 1.
The figure shews (1) The mode of formation of a visceral cleft without any
involution of the external skin. (2) The head-cavity in the arch and its
situation in relation to the aortic arch.
Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 13,
considerably magnified. The specimen was prepared by removing the nasal
pit, flattening it out and mounting in glycerine after treatment with
chromic acid. It shews the primitive arrangement of the Schneiderian folds.
One side has been injured.
Figs. 15_a_ and 15_b_. Two longitudinal and vertical sections through the
head of a Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3.
15_a_ is the most superficial section of the two. It shews the constitution
of the seventh and fifth nerves, and of the intestinal branch of the vagus.
The anterior branch of the seventh nerve deserves a special notice.
15_b_ mainly illustrates the dorsal commissure of the vagus nerve (_com_)
continuous with the dorsal commissures of the posterior root of the spinal
nerves.
Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus
embryo belonging to the end of stage K. Zeiss a, ocul. 1.
16_a_ passes through the median line of the brain and shews the
infundibulum, notochord and pituitary body, etc.
The pituitary body still opens into the mouth, though the septum between
the mouth and the throat is broken through.
16_b_ is a more superficial section shewing the head-cavities _pp_ 1, 2, 3,
and the lower vagus commissure.
PLATE 16.
COMPLETE LIST OF REFERENCE LETTERS.
_auv._ Auditory vesicle. _cb._ Cerebellum. _cer._ Cerebral hemispheres.
_ch._ Notochord. _cin._ Internal carotid. _ft._ Fasciculi teretes. _in._
Infundibulum. _lv._ Lateral ventricle. _mb._ Mid-brain, or optic lobes.
_md._ Medulla oblongata. _mn._ Mandible. _ol._ Olfactory pit. _oll._
Olfactory lobe. _op._ Eye. _opn._ Optic nerve. _opth._ Optic thalamus.
_pc._ Posterior commissure. _pcl._ Posterior clinoid. _pn._ Pineal gland.
_pt._ Pituitary body. _rt._ Restiform tracts. _tv._ Tela vasculosa of the
roof of the fourth ventricle. iv._v._ Fourth ventricle. vii. Seventh nerve.
_x._ Rudiment of septum which will grow backwards and divide the unpaired
cerebral rudiment into the two hemispheres.
Figs. 1_a_, 1_b_, 1_c_. Longitudinal sections of the brain of a Scyllium
embryo belonging to stage L. Zeiss a, ocul. 1.
1_a_ is taken slightly to one side of the middle line, and shews the
general features of the brain, and more especially the infundibulum (_in_)
and pituitary body (_pt_).
1_b_ is through the median line of the pineal gland.
1_c_ is through the median line of the base of the brain, and shews the
notochord (_ch_) and pituitary body (_pt_); the latter still communicating
with the mouth. It also shews the wide opening of the infundibulum in the
middle line into the base of the brain.
Fig. 2. Section through the unpaired cerebral rudiment during stage O, to
shew the origin of the olfactory lobe and the olfactory nerve. The latter
is seen to divide into numerous branches, one of which passes into each
Schneiderian fold. At its origin are numerous ganglion cells represented by
dots. Zeiss a, ocul. 2.
Fig. 3. Horizontal section through the three lobes of the brain during
stage O. Zeiss a, ocul. 2.
The figure shews (1) the very slight indications which have appeared by
this stage of an ingrowth to divide the cerebral rudiment into two lobes
(_x_): (2) the optic thalami united by a posterior commissure, and on one
side joining the base of the mid-brain, and behind them the pineal gland:
(3) the thin posterior wall of the cerebral rudiment with folds projecting
into the cerebral cavity.
Figs. 4_a_, 4_b_, 4_c_. Views from the side, from above, and from below, of
a brain of Scyllium canicula during stage P. In the view from the side the
eye (_op_) has not been removed.
The bilobed appearance both of the mid-brain and cerebellum should be
noticed.
Fig. 5. Longitudinal section of a brain of Scyllium canicula during stage
P. Zeiss a, ocul. 2.
There should be noticed (1) the increase in the flexure of the brain
accompanying a rectification of the cranial axis; (2) the elongated pineal
gland, and (3) the structure of the optic thalamus.
Figs. 6_a_, 6_b_, 6_c_. Views from the side, from above, and from below, of
a brain of Scyllium stellare during a slightly later stage than Q.
Figs. 7_a_ and 7_b_. Two longitudinal sections through the brain of a
Scyllium embryo during stage Q. Zeiss a, ocul. 2.
7_a_ cuts the hind part of the brain nearly through the middle line; while
7_b_ cuts the cerebral hemispheres and pineal gland through the middle.
In 7_a_ the infundibulum (1), cerebellum (2), the passage of the restiform
tracts (_rt_) into the cerebellum (3), and the rudiments of the tela
vasculosa (4) are shewn. In 7_b_ the septum between the two lobes of the
cerebral hemispheres (1), the pineal gland (2), and the relations of the
optic thalami (3) are shewn.
Figs. 8_a_, 8_b_, 8_c_, 8_d_. Four transverse sections of the brain of an
embryo slightly older than Q. Zeiss a, ocul. 1.
8_a_ passes through the cerebral hemispheres at their junction with the
olfactory lobes. On the right side is seen the olfactory nerve coming off
from the olfactory lobe. At the dorsal side of the hemispheres is seen the
pineal gland (_pn_).
8_b_ passes through the mid-brain now slightly bilobed, and the opening
into the infundibulum (_in_). At the base of the section are seen the optic
nerves and their chiasma.
8_c_ passes through the opening from the ventricle of the mid-brain into
that of the cerebellum. Below the optic lobes is seen the infundibulum with
the rudiments of the sacci vasculosi.
8_d_ passes through the front end of the medulla, and shews the roots of
the seventh pair of nerves, and the overlapping of the medulla by the
cerebellum.
PLATE 17.
COMPLETE LIST OF REFERENCE LETTERS.
vii. _a._ Anterior branch of seventh nerve. _ar._ Anterior root of spinal
nerve. _auv._ Auditory vesicle. _cer._ Cerebrum. _ch._ Notochord. _ch._
Epithelial layer of choroid membrane. _gl._ Glossopharyngeal nerve.
vii. _hy._ Hyoid branch of seventh nerve. _hym._ Hyaloid membrane. _ll._
Lateral line. v. _mn._ Ramus mandibularis of fifth nerve. vii. _mn._
Mandibular (spiracular) branch of seventh nerve. v. _mx._ Ramus maxillæ
superioris of fifth nerve. _nl._ Nervus lateralis. _ol._ Olfactory pit.
_op._ Eye. v. _opth._ Ramus ophthalmicus of fifth nerve. _pch._ Parachordal
cartilage. _pfal._ Processus falciformis. _pp._ Head cavity. _pr._
Posterior root of spinal nerve. _rt._ Retina. _sp._ Spiracle. v. Fifth
nerve. vii. Seventh nerve. _vc._ Visceral cleft. _vg._ Vagus nerve.
_vg.br._ Branchial branch of vagus. _vgcom._ Commissure uniting the roots
of the vagus, and continuous with commissure uniting the posterior roots of
the spinal nerves. _vgr._ Roots of vagus nerves in the brain. _vgin._
Intestinal branch of vagus. _vh._ Vitreous humour.
Fig. 1. Diagram of cranial nerves at stage L.
A description of the part of this referring to the vagus and
glossopharyngeal nerves is given at p. 426. It should be noticed that there
are only five strands indicated as springing from the spinal cord to form
the vagus and glossopharyngeal nerves. It is however probable that there
are even from the first a greater number of strands than this.
Fig. 2. Section through the hinder part of the medulla oblongata, stage
between K and L. Zeiss A, ocul. 2.
It shews (1) the vagus commissure with branches on one side from the
medulla: (2) the intestinal branch of the vagus giving off a nerve to the
lateral line.
Fig. 3. Longitudinal and vertical section through the head of a Scyllium
embryo of stage L. Zeiss a, ocul. 2.
It shews the course of the anterior branch of the seventh nerve (vii.);
especially with relation to the ophthalmic branch of the fifth nerve
(v. _oth_).
Figs. 4_a_ and 4_b_. Two horizontal and longitudinal sections through the
head of a Scyllium embryo belonging to stage O. Zeiss a, ocul. 2.
4_a_ is the most dorsal of the two sections, and shews the course of the
anterior branch of the seventh nerve above the eye.
4_b_ is a slightly more ventral section, and shews the course of the fifth
nerve.
Fig. 5. Longitudinal and horizontal section through the hind-brain at stage
O, shewing the roots of the vagus and glossopharyngeal nerves in the brain.
Zeiss B, ocul. 2.
There appears to be one root in the brain for the glossopharyngeal, and at
least six for the vagus. The fibres from the roots divide in many cases
into two bundles before leaving the brain. Swellings of the brain towards
the interior of the fourth ventricle are in connection with the first five
roots of the vagus, and the glossopharyngeal root; and a swelling is also
intercalated between the first vagus root and the glossopharyngeal root.
Fig. 6. Horizontal section through a part of the choroid slit at stage P.
Zeiss B, ocul. 2.
The figure shews (1) the rudimentary processus falciformis (_pfal_) giving
origin to the vitreous humour; and (2) the hyaloid membrane (_hym_) which
is seen to adhere to the retina, and not to the vitreous humour or
processus falciformis.