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The anatomy of a 7.8 MM. pig embryo

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THE ANATOMY O F A 7.8 M I L P IG EMBRYO
FRED W. THYNG’
From the Department of Comparative Anatomy of the Harvard Medical School,
and The Department of Anatomy, Northwestern University Medical School
THREE F I U U R E S ~
Graphic reconstructions of a 12 mm. pig embryo have been
published by Dr. F. T. Lewis (’03) and used in Minot’s “Laboratory Text-book of Embryology. ” These reconstructions have
been exceedingly helpful to the student in acquiring an understanding of embryology. Therefore it seemed desirable to have
similar reconstructions of a still younger stage. At the suggestion
of Prof. C. S. Minot I have made a series of reconstructions to
show the anatomy of a 7.8 mm. pig embryo.
The drawings were made from transverse sections by the modified graphic reconstruction method of His. The surface modeling
has been inferred from a study of the sections with the exception
of a few instances when wax models were made to aid in its interpretation.
EXTERNAL FEATURES
I n contour this embryo resembles closely the one represented
by Keibel (’07) in fig. 15, Taf. 11. There are four regions of
bending or flexures, the cephalic, the cervical, the dorsal and the
sacral. The cephalic is in the region of the mesencephalon and
causes the anterior portion of the brain to be bent at an acute
angle to the posterior. The cervica.1 occurs in the region of the
1 This investigation has been aided by a Bullard Fellowship established at
Harvard Medical School in the memory of John Ware.
* The reconstruction represented in fig. 1 of this paper is used in the second
edition of Prof. Minot’s “Laboratory Text-book of Embryology.” Since this
plate was made several minor changes have been made in the drawing.
THEANATOMICALRECORD,
VOL. 5 , No.1
18
FRED W. THYNG
neck, where the spinal cord joins the brain. It is at an obtuse
angle. The dorsal occurs approximately on a level with the middle
of the anterior limb bud. It also forms an obtuse angle causing
a convexity of the back. The sacral is in the caudal region and
is more nearly at a right angle.
Four branchial arches are observed in fig. 3, separated by ectodermal grooves, the branchial clefts. The fourth is seen at the
bottom of a pit (Sxer.) which represents the developing cervical
sinus. I n size the branchial arches decrease from anteriorly
posteriorly. The maxillary processes of the first arch have united
on either side with the lateral nasal and globular processes so that
the nasal pits are now shut off from the oral cavity. The nasal
fossae are, however, separated by an exceptionally broad median
nasal process.
The ectodermal invagination on either side to form the lens
of the eye has just closed.
The great development of the heart causes a conspicuous swelling of the ventral body wall directly under the branchial arches.
The umbilical cord is comparatively short and bends to the
right. The limb buds are conspicuous structures.
T H E ALIMEPU’TARY CANAL AND ITS APPENDAGES
Mouth. The mouth ( M . ) shown in figs. 1 and 2 is wide and
slitlike. It is bounded posteriorly by the mandibular processes
(Mdb.) of the first arch, and anteriorly by the median nasal
process united on either side with the maxillary process. I n the
dorsal wall of the oral cavity there is a distinct longitudinal furrow
which leads to the opening of the hypophysis.
Hypophysis. The hypophysis ( H y p . ) is a flattened diverticulum
from the ectoderm of the oral cavity closely applied to the ventral
surface of the brain. Its distal end is wider than its proximal
part, but does not show any trace of forking. The oral membrane
has disappeared except for a small remnant attached immediately
posterior to the outlet of the hypophysis.
Pharynx. The pharynx is dorso-ventrally compressed and
has its greatest width in the region of the mouth. It gradually
ANATOMY OF A
7.8
MM. PIG EMBRYO
19
becomes narrower as it extends posteriorly t o its bifurcation into
the oesophagus (Oe.) and trachea (Tr.) d little above the point
of bifurcation it is bent upon itself at an angle corresponding to
the neck-bend of the embryo. A little caudal to the hypophysis
in the median line occurs the most anterior of the entodermal
structures. It is a small conical diverticulum (Seessel's pocket,
S. P . ) resembling somewhat t'he hypophysis. Still farther caudal
the median wall of the pharynx presents another conspicuous
projection (2). The development of this structure has been
followed by Mrs. Gage ('05),who concludes that it represents the
bursa pharyngea identified by Killian ('88). I n the anterior
ventral wall of the pharynx there is a deep conspicuous groove
partially separating the ventral ends of the mandibular processes
of the first visceral arch. Immediately posterior to this median
oral groove is the small rounded elevation, the tuberculum impar (7'. i.), which has been said, (His, '85) t o form the tip and a
large portion of the body of the tongue. Some investigators,
(Kallius '01, Hammar ' O Z ) , however, assert that later in development a more extensive elevation forms from the medial dorsal
surface of the mandibular processes, and that it is this which forms
the greater poition of the anterior part of the tongue. Posteriorly the tuberculum impar is fused to fhe converging ventral
ends of the descending hyoid arches which form the anterior
part of a median elevation slightly higher than that of the tuberculum impar. The original separation of the two is now indicated by a slight V-shaped groove. The apex of this groove
marks the place of origin of the median thyroid (Th. med.).
The ventral ends of the descending third visceral arches have
also converged toward the median line, and have fused with the
median elevation of the second arches to form a conspicuous
posterior part, the epiglottis (Epgl.) . The remaining anterior
portion of the elevation contributes according to His ('85) to
the development of the posterior part or root of the tongue.
Behind the epiglottis and at a much lower level, the internal
wall of the pharynx presents a slight median groove bounded laterally by the arytenoid ridges which represent the lateral portions
of the furcula described by His ('85). This groove leads pos-
20
FRED W. THYNG
teriorly along the ventral wall of the pharynx to the glottis, the
slitlike opening of the larynx into the pharynx.
An approximation of the entoderm of the arytenoid folds occurs
in the region of the larynx, Thereby two passages for a distance
are formed, a broader dorsal channel and a ventral slitlike one,
which leads into the trachea. A little more posteriorly, the dorsal
passage forks, one channel passing into the oesophagus and the
other into the trachea.
Pharyngeal pouches. The pharyngeal pouches are shown in
fig. 2. There are four on either side. As described by Hammar
('02) and confirmed by Fox ('08) each possesses a dorsal and a
ventral diverticulum. I n the 7.8 mm. pig embryo all are in contact
with the ectoderm forming closing plates, except the fourth.
Anteriorly the pharynx is wide and much compressed dorsoventrally. From the anterior lateral border the first or auditory
pouch (Ph. P. 1.) arises. Its dorsal diverticulum is the most
conspicuous portion. It is a wing-like projection extending
obliquely upward and outward, ending in a pointed process.
There is thus formed a prominent ridge extending from the pointed
tip of the dorsal process downward, inward and forward to the
oral epithelium between the hypophysis and the angle of the
mouth. This has been named by Moldenhauer ('77) the sulcus
tubo-tympanicus. The contact of this pouch with the ectoderm
is made by a lateral ridge extending from the dorsal pointed
process posteriorly downward for about three-fourths of its distance. Below the point of contact there is a small ventral diverticulum less developed than the corresponding structure of the other
pouches, but causing a distinct ventral ridge on the floor of the
pharynx extending inward and backward toward the median line.
The pharynx is also quite wide in the region of the second pouch
(Ph. P. 2.). Immediately posterior t o this it retreats considerably
toward the median line. From the lateral and ventral border
of this widened portion of the pharynx the second pouch develops.
Its dorsal diverticulum somewhat resembles that of the auditory
pouch being flat and extending upward and outward. It terminates in two short pointed processes leaving a slight indentation
down over which passes the tympanic branch ( N . tymp., Jig. 3.)
ANATOMY OF A
7.8
MM. PIG EMBRYO
21
of the glossopharyngeal nerve. Between this diverticulum and
the one anterior there is a V-shaped notch. The surface of contact
to form the closing plate of this pouch is very extensive as shown
by Fox ('08). It is formed by a deep plate-like fold of the entoderm
jutting out at almost right angles to the axis of the pharynx. The
border of contact recedes from the ventral border of the dorsal
diverticulum medianly to its extreme lower tip. This fold thus
produces a prominent ventral diverticulum extending down between the second and third visceral arches with its extreme tip
pointed medianly. Between the ventral tips of the diverticula
of the two sides is the median thyroid (Th. med.) a small branching
mass of entoderm.
The third pouch, likewise, arises from the latero-ventral wall
of the pharynx. It is plate-like and nearly parallel with the second.
Its dorsal diverticulum is smaller than the two preceding, but
its ventral diverticulum, which forms the thymus gland, is more
prominent. It is a deep fold from the entoderm. Its upper lateral
border is nearly parallel to the corresponding border of the third.
Medially they tend to converge. A little more than half of its
border reaches the ectoderm. Beyond, the ventral part is free.
Immediately behind the third pouch the pharynx recedes abruptly
toward the median line.
The fourth pharyngeal pouch (Ph. P. 4.) conspicuously smaller
than the preceding joins the pharynx a little anterior to its
bifurcation into the oesophagus and trachea. It possesses a
rounded dorsal diverticulum, attached to the pharynx, somewhat
constricted off from a more ventral tubular portion which extends
posteriorly and somewhat dorsally. This posterior ventral port,ion
is comparable with the ventral process of the other pouchesas
emphasized by Fox ('08). From the ventral surface of the fourth
pouch a prominent ridge extends across the ventral wall of the
pharynx to meet the corresponding one from the other side.
Lungs. Above the cardiac end of the stomach the trachea
bifurcates to form two short bronchi, fig. 1. Each entodermal
bronchus ends in a symmetrical bilobed enlargement, the lung.
The left bronchus has been cut away in the figure. The trachea
just above its point of bifurcation shows an indication 01 the
developing eparterial bronchus (not represented in the drawing).
22
FRED
W. THYNG
Liver. The liver (hepar) is incompletely divided into four
lobes. Two lobes, a dorsal and a ventral, appear on either side.
I n fig. 1 a median sagittal section of the liver is represented. In
fig. 2 it is represented as cut more to the right, and here a better
idea is obtained of its comparatively large size.
Stomach. The stomach (St.) has so revolved on its axis that
its dorsal border nearly faces toward the left. Its previous ventral border is now toward the right. Viewed from the left and
ventrally it has an oval shape, tapering at either end, while
from the left and dorsally it is very much flattened hardly thicker
than the oesophagus. Just below the entrance of the oesophagus,
the cardiac end presents a small dorsally directed elevation corresponding to the prominent pouch of the stomach of the 12 mm.
pig embryo (Lewis '03, fig. 3).
Gall bladder and pancreas. Ventral to the pyloric end of the
stomach is the hepatic diverticulum from which project several
cords of cells connecting with the liver trabeculae, one of which
will eventually form the hepatic duct. The distinct pouch at
the distal ventral end of the hepatic diverticulum together
with the more constricted intermediate portion represent the
gall bladder and the cystic duct. The hepatic diverticulum
opens into the right side of the duodenum. In a younger pig
embryo (Thyng '08, fig. l), this opening was on the ventral side.
The change has occurred by a revolving of this portion of the
intestine, thus causing the hepatic diverticulum to be bent a t
nearly a right angle.
The ventral pancreas (Panc. Y.) joins the hepatic diverticulum
near its opening into the duodenum. It is an elongated budding
mass of entoderm, extending on the right side of the duodenum
ventral to the portal vein (V. p . ) . The dorsal panereas (Panc. d.)
is larger than the ventral. It joins the duodenum on its dorsal
side, posterior to the opening of the hepatic diverticulum. It
branches in the mesenchyme of the greater omentum, sending
a. posterior subdivision to the right and ventrally which overhangs
the portal vein. The ventral and dorsal pancreas will later anastomose on the right side of this vein. Neither the dorsal nor the
ventral pancreas as yet possesses an open outlet. In the pig the
ANATOMY OF A
7. 8
MM. PIG EMBRYO
23
duct of the dorsal pancreas persists, the ventral becomes obliterated (Stoss '91).
Intestine. From the dorsal pancreas the intestine (Int.)
extends in the dorsal mesenteiy into the umbilical cord (Urn.
C . ) where it receives the yolk stalk (Yk. 8.). It then returns
to the body and extends posteriorly into the cloaca (CZ.).Beyond
the cloaca it is continued out into the tip of the tail as the tail
gut (post-anal gut). Posterior to the cloaca it is slender except
near its termination where its lumen distinctly enlarges. The
tail gut very early disappears. It has become obliterated in the
12 mm. pig embryo, (Lewis '03, plate 3).
Cloaca. The cloaca (Cl.) represents a dorso-ventrally expanded
portion of the intestinal tract from which the allantois (All.)
extends forward and ventrally into the posterior wall of the
umbilical cord. The allantois is a slender tube as it leaves the
ventral portion of the cloaca. I n the umbilical cord it soon
becomes considerably dilated. Lateral to the origin of the allantois
the cloaca receives the Wolffian ducts. (0.
W.). The anterior
portion of the cloaca to which the Wolffian ducts and allantois
are connected is partly separated from the dorsal intestinal portion
by a fold (uro-rectal) to form the urogenital sinus. In the 12
mm. pig embryo, (Lewis '03, plate 3), the cloaca has become
more extensively separated into its urogenital and rectal divisions.
The ventral wall of the cloaca anastomoses with the adjacent
ectoderm to form a median raphe, the cloaca1 membrane.
URONGENITAL SYSTEM
The large Wolffian ridges or mesonephroi (Mes.) are shown in
part in fig. 2. They consist mainly of the mesonephric tubules
and the associated mesial glomeruli. They begin anteriorly near
the anterior border of the liver and extend almost the entire length
of the body cavity. The posterior portion of each ridge, however,
is occupied only by the large Wolffian duct (D.W . ) which joins
the corresponding antero-lateral portions of the cloaca. The
division of the cloaca into which the ducts empty is nearly separated from the dorsal rectal portion as already described. Just
before the Wolffian ducts open into the urogenital region of the
24
FRED W. THYNG
cloaca, each presents an evagination (Met.) from its dorsal wall,
extending into the mesenchyme above. This evagination, the
primitive pelvis of the metanephros, shows three lobes developing
from its surface, a posterior bud near its union with the Wolffian
duct and two terminal ones.
NERVOUS SYSTEM
Bruin. The longitudinal axis of the brain and spinal cord
share the corresponding flexures already described for the embryo.
The ventral flexure of the floor of the metencephalon is already
quite apparent. The three primary divisions of the brain, prosencephalon, mesencephalon and rhombencephalon, have undergone the following differentiation in the 7.8 mm. pig embryo.
Prosencephalon. The prosencephalon, (figs. 1and 3) is partially
divided into diencephalon (Dien.) and telencephalon (TeZen.)
by a prominent external groove and a corresponding internal
ridge, the velum transversum, (Johnson '09). This ridge extends
from the dorso-median line around the interior of the prosencephalon ending in front of the optic evagination. The cerebral
hemispheres are shallow evaginations from the dorso-lateral
surface of the telencephalon, the first neuromere of the brain.
A small pit in the ventral wall of each hemisphere indicates the
developing olfactory area (rhinencephalon). Ventral to the olfactory depression there is a low broad arch which represents
the corpus striatum. Caudal to this there is another ridge or
arch which overlies the groove leading from the optic cup. I n this
ridge the optic fibres later develop, and so Johnson has named it
the optic ridge. It is separated from the developing corpus
striatum by a groove, the praeoptic recess which crosses the thin
lamina terminalis and extends out into the optic stalks.
Diencephalon. The diencephalon is bounded from the mesencephalon (Mesen.) by an internal ridge extending transversely
across the brain toward the tuberculum posterius. This division
is narrow, but considerably dilated dorso-ventrally.. A large
concavity marks each dorso-lateral wall. In the anterior part of
each latero-ventral wall is the optic evagination. I n the median
ANATOMY OF A
7.8 MM.
PIG EMBRYO
25
line the brain wall is thickened just behind the tip of the oral
hypophysis, to form its nervous portion or posterior lobe. Still
farther caudal there is a slight depression, the mammillary recess,
causing a corresponding external elevation of the floor of the diencephalon. The number of neuromeres entering into the formation
of diencephalon is questionable. Johnson ('08) gives two neuromeres to this section of the brain.
Mesencephalon. The posterior boundary of the mesencephalon
(Mesen.) is a ridge known as the plica-rhombomesencephalica.
Each dorsal zone of this division presents three concavities or neuromeres on its internal surface, and corresponding swellings on its
external surface. Schultze ('97) found in a pig embryo 2-3 neuromeres in the mesencephalon. Johnson ('09) represents two in
this region of the pig embryo. The,ventral zone of the mesencephalon is thickened causing a prominent external elevation just
anterior to the origin of the oculomotor nerve.
Rhombencephalon. The conventional divisions of the rhombencephalon are not so well differentiated as those previously
described. The isthmus (Isth.) is the constricted portion in the
region of the ylica-rhombomesencephalica. The metencephalon
(Meten.) is the portion immediately following the isthmus, already
considerably dilated to either side and presenting a developing
fold in its ventral wall, the pontine flexure. The myelencephalon
(Myelen.) may be arbitrarily bounded from the metencephalon
by a line passing transversely across the brain near the posterior
extension of the pontal flexure. From here the myelencephalon
extends posteriorly arching over the cervical flexure to merge
with the spinal cord. The part of the metencephalon from which
the cerebellum will later develop does not show any differentiation except a local thickening of the epithelium. The roof
plate is thin and considerably expanded. The lateral walls of
both metencephalon and myelencephalon are indented by nearly
transverse furrows (neuromeres) involving both dorsal and ventral
zones. Six appear in the anterior two-thirds of the rhombencephalon. A large probably composite neuromere is found in
the posterior third (myelencephalon) over the cervical flexure.
Bradley ('04) has described a neuromere occurring in the region of
26
FRED W. THYNG
the cerebellum of the pig embryo. I n younger pig embryos I
have seen neuromeres in the rhombencephalon anterior t o those
here described. The cranial nerves have been found to be intimately associated with the brain wall underlying these neuromeres.
Spinal cord. The spinal cord ( M e d . sp.) shows a differentiation
into dorsal and ventral zones, and also indications of neuromeric
grooves. McClure ('89) called attention to the neuromerei,
Li
myelomeres, ' of the spinal cord.
CRANIAL NERVES
Nn. olfactorius and opticus. Fiber tracts for the olfactory
apparatus, and for the eye have not developed.
N . thalamicus. A small elongated clump of ganglion cells
( N . th.) were found on either side of the diencephalon dorsal to
the corresponding optic vesicle. Between the ganglion and the
optic vesicle passes the nasal branch ( N . nu.) of the ophthalmic
nerve. No connection between the nerve and ganglion was found.
From the dorsal end of the ganglion, fibres could be traced for a
very short distance into the mesenchyma. It probably represents
a portion of the neural crest corresponding to the nervus thalamicus of Miss Platt, which was found by growth of the brain
wall, to lie directly above the eye stalk (Platt '91, pp. 97-98).
It is possible that this ganglion may contribute t o the formation
of the ciliary ganglion which has been found by Dixon ('96), to
develop in this locality. He, however, is inclined t o consider the
ciliary as a sympathetic derivative.
N . oculomotorius. The oculomotor nerve ( N . oc.) arises from the
thick ventral zone of the mesencephalon by several small rootlet,s.
It extends anteriorly, laterally and ventrally to a developing eye
muscle situated lateral t o the infundibular region, posterior to
the eye vesicle and medial to the anterior cardinal vein.
N . troddearis. The rootlets of the trochlearis ( N . troch.) are
small and are not grouped to form a definite bundle. They leave
the brain at thedorsal border of the isthmus and can be traced
only a short distance.
N . trigeminus. The trigeminal nerve ( N . tri.) consists of a
sensory (portio major) and a lateral motor division (portio minor).
ANATOMY OF A
7. 8
MM. PIG EMBRYO
27
It is conspicuous on account of the large semilunar ganglion (G. sl.)
connected with it. It gives rise to three branches, the ophthalmic,
the maxillary and the mandibular. The ophthalmic leaves the
anterior ventral portion of the ganglion and soon gives off a
dorso-laterally directed nerve, the frontal ( N .fr.) which is related
to the ectoderm in the immediate vicinity. A small clump of
cells is connected with the nerve near its termination. I n front
of the right semilunar ganglion, and just above the termination
of the frontal there is another small group of cells. A corresponding group on the left did not exist. Beyond the frontal, the nasal
division ( N .nu.) of the ophthalmic extends anteriorly and dorsally
above the eye stalk.
The maxillary ( N . mz.) arises from a considerable ventral lobe
of the semilunar ganglion, and extends into the maxillary process
of the first arch behind the lachrymal groove. A4s scattered
fibres it can be traced as far ventrally as the optic stalk.
The mandibular ( N . md.) extends, from the posterior ventral
border of the ganglion, laterally and ventrally to the mandibular
process (Mdb.) of the first arch. It can be followed some distance
below the angle of the mouth. It contains the portio minor or
motor part of the trigeminal.
The sensory roots of the trigeminal enter the brain opposite
the first and second of the rhombic neuromeres represented for
this embryo. The motor fibres arise from the neuroblasts situated
in the basal portion of the same neuromeres.
N . abducens. The abducens ( N . abd.) arises from the ventral
zone of the fourth rhombic neuromere directly medial to the
lower end of the otocyst ( V . aud.). That on the right side shortly
after it leaves the brain possesses a small aberrant branch coming
from the region of the glossopharyngeus or vagus. The nerve
extends anteriorly to a dense mass of cells separated from the
semilunar ganglion by the anterior cardinal vein.
N . facialis. The motor roots of the facial arise from neuroblasts in the ventral zone of the third rhombic neuromere.
At their origin the rootlets are somewhat separated, but as
they issue from the metencephalon, they are four or five in
number. They soon converge to a single bundle which passes
28
FRED W. THYNG
along the posterior border of the geniculate ganglion (G. gn.),
and extends posteriorly and laterally along the posterior border
of the first entodermal pouch, in intimate association with a
bundle of sensory fibres issuing from the neuroblasts of the geniculate ganglion. Just behind the closing plate of the first pouch
the associated bundles divide into a posterior and an anterior
division, the facial and the chorda tympani ( N . ch. tymp.) respectively. The facial can be traced for a short distance into the
hyoid arch. The chorda tympani winds around the ventral process
of the first entodermal pouch and enters the mandibular process.
The geniculate ganglion (G. gn.) is an elongated mass of cells.
It extends from the median surface of the dorsal diverticulum
of the first pharyngeal pouch, to which it is for a distance closely
applied, to a level of the middle of the otocyst. Its fibres, the
intermedius, enter the metencephalon in the dorsal zone at the
posterior border of the third rhombic neuromere under cover of
the entering roots of the acustic ganglion (G. acus). From the
ventral tip of the geniculate ganglion a small nerve massed
with cells, passes anteriorly and ventrally along the dorsal median
border of the auditory pouch. This is the great superficial petrosal.
N . acusticus. The ganglion of the auditory nerve (G. acus.)
is an elongated mass of cells closely applied to the middle anterior
median border of the otocyst. I n the middle portion where an
upper group of fibres to the ear will later appear, the ganglion is
fused with the epithelium of the otocyst. From the ventral
end of the ganglion a small nerve extends posteriorly to the epithelium of the lower median portion of the otocyst. According to
Streeter ('08)'this later will innervate the posterior ampulla and
the saccule. The entering fibres from the extreme dorsal part of
the acustic ganglion pass into the dorsal zone of the metencephalon opposite the third rhombic neuromere as two rootlets just
above the entering intermedius.
N . glossopharyngeus. The glossopharyngeal ( N . glos.) like
the vagus to be described presently is characterized by possessing
two ganglia, the ganglion superius (G. sup.), and the ganglion
petrosum (G. petros.). The latter is more sharply outlined than
ANATOMY OF A
7.8
MM. PIG EMBRYO
29
the former-and intimately connected at its lower end with the
posterior border of the dorsal diverticulum of the second pharyngeal pouch, and the adjacent ectoderm. This nerve possesses
both sensory and motor fibers. The more numerous sensory
rootlets are covered with ganglion cells continuous with those
of the superior ganglion. They enter the dorsal zone of the myelencephalon opposite the fifth rhombic neuromere. A ganglion
crest or bridge still connects the superiorganglion with the jugular
ganglion (G.jug.) of the vagus. The motor fibres arise from neuroblasts in the mantle layer of the ventral zone of the fifth rhombic
neuromere. They extend laterally through the marginal velum
of the brain and emerge as a number of small rootlets ventral
to the sensory components. The glossopharyngeal divides distally
sending a branch, the tympanic ( N . tymp.), down over the dorsal
diverticulum of the second pharyngeal pouch into the hyoid
arch. Posterior to the dorsal diverticulum it divides into two
small branches, an anterior, the lingual ramus and a posterior,
the pharyngeal ramus.
N . vagus. The vagus ( N . vag.) possesses a dorsal ganglion the
jugular(G. jug.)and a ventral one, the ganglion nodosum (G. nodos.)
The jugular grtnglion is large and is connected with an extensive
posterior portion, a part of the neural crest, which ends in an
S-shaped group of cells. The cells in the posterior limb of the S
are more compact than those in the anterior limb, and fibres from
its dorsal apex enter the brain as a distinct root. The ganglionic
crest connecting with the jugular, is the undifferentiated representative of several ganglia, companions of groups of the more
ventral roots composing the hypoglossal nerve ( N . hyp.). The
jugular ganglion is also continuous with cells which overlie the
sensory roots quite close to their entrance to the brain. These roots
are here represented diagrammatically. The ganglion nodosum
is also quite elongated, and is covered externally to a considerable
extent by vagus fibres. It is connected intimately with the ectoderm of the posterior wall of the fourth ectodermal groove.
The vagus contains both sensory and motor fibres. The sensory
fibres arise from cells of the jugular ganglion, including the ganglionic crest. The motor components arise in part from neuroblasts
30
FRED W. THYNG
in the dorso-lateral part of the ventral zone of the sixth rhombic
neuromere. These fibres leave the brain under the entering
sensory roots. Another portion of its motor fibres are derived
from the spinal accessory to be described presently. Posterior
to the third pharyngeal pouch the vagus gives off an anterior
branch thc superior laryngeal ( N . lury’g. sup.). Beyond this
it passes behind the pulmonary arch and can be traced for a distance where it is ventral and lateral to the oesophagus. The
auricular and recurrent branches of the vagus are not formed in
this embryo.
N . uccessorius. The spinal accessory ( N . acc.) is derived from
small rootlets issuing from the dorso-lateral part of the ventrai
zone of the myelencephalon and the spinal cord as far posteriorly
as the sixth cervical ganglion. Caudally the rootlets issue from
the spinal cord below the main trunk of the accessory and much
nearer the ventral roots than they do more anteriorly, showing
somewhat of a gradual transition into ventral roots. Lubosch
(’99) who has made a comparative study of the accessory nerve,
concludes that as a general rule its more posterior rootlets are
nearer the dorsal roots of the spinal ganglia. Exceptions, however,
to this rule were noted (pp. 530 and 545). The spinal accessory
composed of only a few fibres in its posterior part, gradually
increases in caliber as it extends forward under the spinal ganglia.
It finally turns ventrally along the posterior border of the vagus.
Below the jugular ganglion it becomes closely connected with the
vagus, contributing to its (vagus) motor part. A portion of the
accessory, however, bends dorsally, away from the vagus for the
later innervation of the trapezius and sternomastoid muscles.
N . hypoylossus. The hypoglossal ( N . hyp.) is a composite
motor nerve arising by several rootlets from the ventral part of
the ventral zone of the myelencephalon. These rootlets soon converge to form the main trunk. The corresponding dorsal ganglia
are represented by an undifferentiated portion of the neural crest
which, however, shows indications of ganglion formation in its
posterior part.
ANATOMY OF A
7.8
MM. PIG EMBRYO
31
SPINAL NERVES
Portions of the anterior five cervical nerves (Nn. sp. 1-5.) and
their ganglia are shown in the reconstruction. The ganglia are
still joined together by portions of the neural crest. Ganglion
cells continuous with those of the spinal ganglia cover the sensory
rooblets close to their entrance into the spinal cord.
SENSE ORGANS
The extent of the nasal invagination is represented in fig. 2.
It is a shallow laterally compressed pit ( F . nu.). Its thickened
epithelium does not reach the wall of the oral cavity.
The right optic vesicle (V. op.) with its connecting stalk is
represented in fig. 3. It is cup-shaped on its lateral surface except
for a groove which leaves the cup, and extends along the border of
the stalk toward the brain. The lens is still connected with the
ectoderm by a slender stalk cut across in the drawing.
The otocyst ( V . aud.) is also represented in fig. 3. It is an
oval ectodermal vesicle lying adjacent to bhe lateral surface of
the rhombencephalon. Above it projects the ductus endolymphaticus which arises from its median dorsal surface.
VASCULAR SYSTEAI
Heart. I n fig. 1 the heart is represented as sectioned on the
left side of the median line. The trabeculae of the ventricle here
as also in fig. 2, are represented diagrammatically. The left atrium
is now largely separated from the right by the septum primum
(atrial septum) united ventrally with two thickenings of the
atrial canal. Between these thickenings a small ventral passage
still persists which unites the two atria. It is the interatrialforamen ( F . ia.). A secondary opening in the septum primum the
foramen ovale ( F . 0.) occurs more anteriorly and dorsally.
I n the posterior median part of the left atrium there is a small
opening marking the outlet of the single pulmonary vein ( V.
pul.). Ventral to this vessel the left common cardinal vein is cut
32
FRED W. THYNG
obliquely. Between the left atrium and the corresponding ventricle a median antero-posterior ridge formed by the union of
the thickenings of the artial canal, represent the anlage of the
right portion of the bicuspid valve. Immediately ventral to this
ridge there is a large opening, the interventricular foramen
(F. iv.) bounded ventrally by the free edge of the ventricular
septum. Through this opening the two ventricles communicate.
Later in development it comes to open into the ventral aorta
(A0.v.) by the union of the aortic and ventricular septa. The
aortic septum is already indicated in the aortic bulb, fig. 2, by
two folds or ridges, a and b. The ventricular septum connects
with the left fold ( b ) while the one on the right (a) connects with
the right ventricular wall just anterior to the atrio-ventricular
foramen. The left ventricle already has a much thicker wall
of muscular trabeculae than the right.
I n fig. 2. the heart is sectioned on the right of the median line.
Into the right atrium (At. d.) opens the sinus venosus (8.v.)
between its two valves which unite anteriorly and extend along
the atrial wall as a ridge, the septum spurium. The atrium opens
into the corresponding ventricle by the right atrio-ventricular
foramen. On the right side of this foramen is figured the anlage
which contributes t o the formation of the tricuspid valve. The
right ventricle opens into the bulbus arteriosus. This outlet
is being subdivided by the formation of the aortic septum as
already described. The permanent outlet will be through the
pulmonary trunk which as indicated in the figure, leaves the
antero-ventral part of the ventricle and passes dorsally and medially across the first portion of the aortic channel.
Arteries. The arteries are figured in figs. 1 and 2. In fig. 2
the ventral aorta (Ao. v.) after leaving the heart is seen to give
off on the left five aortic arches one to each visceral arch. Their
relation to the pharyngeal pouches is clear. The slender first
and second arches on either side arise from a common trunk as
the ventral aorta bifurcates just behind the median thyroid
(Th. med.). In fig. 1 the corresponding aortic arches on the right
are represented as showing through the pharyngeal wall. The
intermediate portion of the second arch on the right has become
ANATOMY OF -4
7.9
MM. PIG EMBRYO
33
obliterated. The most posterior or pulmonary arches on account
of the developing aortic septum leave the aorta by a short common
trunk. A little above the bifurcation of this trunk each pulmonary arch gives off posteriorly a small pulmonary artery ( A .pul.).
The right pulmonary artery arises from the corresponding arch
by two small branches. The aortic arches of eachside extend around
the lateral border of the pharynx and join the descendingaorta (Ao.
desc.) of the corresponding side. The two descending aortae extend
caudally and unite near the cardiac end of the stomach to form the
median dorsalaorta (Ao. med. dor.). Each descending aorta from
where it, is joined by the first aort,icarch is continued anteriorly into
the maxillary arch posterior to the optic stalk. This represents a
portion of the internal carotid artery ( A . car. int.) . It gives off
a branch above the optic stalk which represents the anterior
cerebral artery. It is here joined by the posterior communicating
branch ( A . corn. post.) of the basilar ( A . bas.) which gives off in
front of the oculomotor nerve the posterior cerebral. Caudal to
the posterior communicating branches the anterior part of the
basilar is a single vessel and gives off several lateral branches,
but along the greater part of the pontine bend it is represented
by two vessels intimately connected by cross branches. From
each of these vessels numerous branches extend laterally. Posteriorly these two longitudinal vessels merge with a complicated
system of capillaries spreading along the ventral and lateral
surfaces of the spinal cord.
Six intersegmental arteries arise from each descending aorta
(Ao. desc.). From the median dorsal aorta corresponding pairs
are given off at regular intervals along its course to the tail. Those
on the left are represented as cut away. Each intersegmental
artery extends from the aorta dorsally and laterally to the lateral
surface of the spinal cord alongside the corresponding spinal
nerve. From the seventh intersegmental artery the most anterior
of those arising from the median aorta, a lateral branch, the subdavian extends into the upper iimb-bud. A series of ventrolateral branches from either side of the aorta extend to the
glomeruli of the corresponding mesonephros.
THEANATOMICAL
REconD,
VOL.
5, N O . 1
34
FRED W. 'THYNG
The most anterior of the median ventral branches of the aorta
is the coeliac axis ( A . coel.) which arises opposite the pyloric
end of the stomach. The omphalo-mesenteric (Aa. omp. mes.)
is represented by three median branches which anastomose in
several places. In their course to the yolk sac, they cross the
intestine on the right and give off branches to the mesentery.
The two anterior roots of the omphalo-mesentericwilllater atrophy
the posterior persisting to form the superior mesenteric artery.
In a human embryo of 5 mm. Tandler ('03), found that the omphalo-mesenteric artery was represented by four or five median
branches from the aorta. These united to form a ventral longitudinal anastomosis extending downward in front of the aorta.
Later in development all these roots disappeared with the exception of the posterior one which formed the mainstemof thesuperior
mesenteric artery.
Each common iliac ( A . I!. corn. d. and s.) arises from the aorta
by two short rootlets (cut off on the left) and extend laterally
and ventrally to the region of the posterior limb-bud to which
it gives off a branch, the external iliac. It then continues forward
and passes into the umbilical cord as the umbilical branch. The
original vessels which connected the umbilical arteries with the
latero-ventral wall of the aorta are now represented by a single
trunk ( A . mes. inf.)just anterior to the origin of the iliacs. This
soon divides after leaving the aorta into right and left branches
each of which can be traced ventrally through a strand of tissue
passing across the posterior part of the coelom to the corrtsponding umbilical branch. This vessel which earlier represented a
part of the umbilical arteries apparently becomes the inferior
mesenteric artery of the adult.
Veins. The vitelline vein ( V . vi., fig. I.) arising on the surface
of the yolk sac passes into the mesentery and extends parallel
with the first limb of the intestinal loop, finally crossing it on the
left a little posterior to the pancreas. Here it is joined by another
vein of about the same caliber, the superior mesenterie ( V . mes.
sup.) which arises in the mesenchyma of the mesentery by numerous tributaries. The common trunk known as the portal vein (V.
p.) divides into two branches corresponding to the original right and
ANATOMY OF A
7.8
MM. PIG EMBRYO
35
leftvitellinevessels. The left branch is small. It passes on theleft
of the dorsal pancreas where it is resolved into a plexusof small
vessels extending in the mesenchyma of the mesogastrium. It
originally extended on to the liver. The right branch extends anteriorly between the ventral pancreas below and a subdivision of
the dorsal pancreas above. It then bends to the right and enters
the liver ( V . p., fig. 2). In the liver it opens into thesinusoids
which represent subdivisions of the vitelline veins, (Minot,’ 00).
The umbilical veins secondarily have acquired an opening into
the hepatic sinusoids. The right (V. urn. d.) represented in fig.
2, anastomoses with the larger one on the left within the cord.
It then passes anteriorly through the somatopleure of the ventral
body wall, receiving tributaries in its course from the posterior
limb-bud and body wall. It finally enters the liver and breaks
up into the hepatic sinusoids. The left umbilical upon entering
the liver passes, as a large channel, the ductus venosus (D. v.)
through the sinusoids toward &heheart.
From a plexus of veins overlying each dorso-lateral surface
of the prosencephalon from which the superior longitudinal sinus
will later develop a lateral branch extends on either side across
the diencephalon. The one on the right is here represented. Dorsally this is joined by several veins coming from the region of the
mid-brain, isthmus and anterior part of metencephalon. Ventrally it is joined by the ophthalmic vein ( V . oph.) which arises
from a plexus in the mesenchyma near the nasal epithelium, and
coursing along under the lachrymal groove receives branches
from the region of the eye. The part of the anterior cardinal
(V. card. ant.) formed by these branches is internal to the semilunar ganglion and has been called the cavernous sinus. The
anterior cardinals then extend posteriorly, lateral to the ventraI
wall of the rhombencephalon and outside the otocyst and cranial
nerves with the exception of the hypoglossal which as it passes
forward crosses the vein on its outer side. Behind the otocyst
each anterior cardinal receives a Iarge branch proceeding from
a plexus of capillaries overlying the lateral surface of the myelencephalon. Posteriorly it passes to the common cardinal vein
( V . curd. corn. G?.).Just anterior to the common cardinal its dorsal
36
FRED W. THYN‘G
wall is connected with an anastomosing plexus of vessels with
which the most anterior intersegmental veins unite. While ventrally it receives the linguo-facial vein (V. Zing.-fuc.) (Grosser
‘01, Lewis ’03 and ’09) arising on the corresponding side of the
branchial arches. Posterior to the opening of the linguo-facial
vein, from the median side of each anterior cardinal, small branches
extend in front of the trachea. These eventually will form the
vena anonyma sinistra.
The anterior and posterior portions of the right posterior cardinal
( V . card. post.) are well defined. In the region of the mesonephric
tubules it has been divided into Wolffian sinusoids (Minot ’00),
and its course along the dorsal border of the Wolffian bodyhas
been interrupted in places. Into the ventral wall of the persisting
portions of the vein enter numerous sinusoids which extend around
the lateral side of the metanephros. Four of these are represented
in the drawing. In a similar manner a series of sinusoids earlier
passed around the mesial side of the metanephros and connected
with the posterior cardinal. Between these median sinusoids there
now has formed on either side a longitudinal anastomosis,
the subcardinal vein ( V . scard. d.), (Lewis ’02). The dorsal portions of some of the median sinusoids on the right are seen, either
connecting the subcardinal and posterior cardinal or upon the
interruption of the latter, taking over to the sub-cardinal the thus
isolated parts with the entering intersegmental veins. The ventral
portions of the median sinusoids have in part atrophied. The subcardinal on the Ieft connects anteriorly with the posterior cardinal.
The subcardinal on the right which forms a part of the inferior
vena cava has made a secondary connection with the ductus
venosus by a “tapping of the hepatic sinusoids” (Lewis, ’02).
Its connection with the corresponding posterior cardinal has been
lost, and its anterior extremity is now a small branch arising from
its dorsal wall near where it bends toward the liver. At the lower
border of the liver a small sinusoidal connection with the subcardinal is seen. Between the right and left subcardinals twelve
places of anastomosis were found. The four posterior, situated
a little below the posterior or permanent root of the omphalo-
ANATOMY OF A
7.8
MM. PIG EMBRYO
37
mesenteric artery, probably coalesce to lorm the large renal
anastomosis represented for the 12 mrn. pig embryo by Lewis.
It also seems probable that in the pig between the subcardinal
and posterior cardinal divisions of the inferior vena cava there is
formed an intermediate portion also from Wolffian sinusoids, but
dorsal to the mesonephric arteries and medial to the posterior
cardinal. An indication of this vessel is shown united anteriorly
with the subcardinal by a broad connection but with slender cross
connections posteriorly. The posterior cardinal in this region is
slender and its continuity in places is lost. The right subclavian
vein ( V . scl. d.) arises from the posterior cardinal between the
seventh and eighth intersegmental vessels.
A large vein from the liver, the vena hepatica communis (V.
hep. corn.) extends anteriorly, and unites with the right and left
common cardinal veins to form the sinus venoms which opens
into the right atrium as already described.
FRED W . THYNG
BIBLIOGRAPHY
BRADLEY,
0. C. 1904 Neuromeres of the rhombencephalon of the pig. Review
of Neurology and Pyschiatry, vol. 2, pp. 625-635.
DIXON,A. F. 1896 On the development of the branches of the fifth cranial
nerve in man. Trans. of the Roy. Dublin SOC.,vol. 6, PIS. 1-2, pp.
19-76.
Fox, H. 1908 The pharyngeal pouches and their derivatives in the mammalia.
Amer. Jour. Anat., vol. 8. pp. 187-251.
GAQE,SUSANNA
P. 1905 A three weeks human embryo, with especial reference
to the brain and the nephric system. Amer. Jour. Anat., vol. 4,
pls. 1-5, pp. 409-443.
GROSSER,0. 1901 Zur Anatomie und Entwickelungsgeschichte des Gefasssystems der Chiropteren. Anat. Hefte, Heft, 55.
HAMMAR,
J. A. 1902 Algemeine Morphologie der Schlundspalten beim Menschen. Entwickelung des Mittelohrraumes und des ausseren GehBrganges. Arch. f. mik. Anat., Bd. 59, Tafel. 26 -29, S. 271-628.
HIS, W.
1880-1885 Anatomie menschlicher Embryonen.
Leipzig.
Text and Atlas.
JOHNSON,
J. B. 1909 The morphology of the forebrain vesicle in vertebrates.
Jour. Comp. New. Psych. vol. 19, pp. 457-540.
KALLIUS,E. 1901 Beitrage zur Entwickelung der Junge, Verh. der Anat.,
Ges. 15.
KDIBEL,F. 1897 Nomentafel I Zur Entwickelungs-geschichte des Schweines.
Jena, S. 1-113.
KILLIAN,G. 1888 Uber die Bursa und Tonsilla pharyngea.
vol. 14, Tafel 25-26, S. 618-711.
Morph. Jahrbuch.
LEWIS, F. T. 1902 The development of the vena cava inferior. Amer. Jour.
Anat., vol. 1, pp. 229-224.
1903 The gross anatomy of a 12-mm. pig.
2, pls. 1-2, pp. 211-225.
h e r . Jour. Anat., vol.
1909 On the cervical veins and lymphatics in four human embryos.
Amer. Jour. Anat., vol. 9.
LUBOSCH, 1899 Vergleichend-anatomischeUntersuchungen uber den Ursprung.
und die Phylogenese des N. Accessorius, Willisu. Arch. f . mik. Anat.,
Bd. 54, Tafel. 27. S. 514-602.
MCCLURE,C. T. W. 1889 The primitive segmentation of the vertebrate brain.
Zoo]. Anz., vol. 12.
MINOT,C. S. 1900 On a hitherto unrecognized form of blood circulation. Proc.
of Bost. SOC.of Nat. Hist., vol. 29, pp. 185-215.
1903 A Laboratory text-book of embryology. Philadelphia.
ANATOMY OF A
7.8
MM. PIG EMBRYO
39
MOLDENHAUER,
W. 1877 Die Entwicklung des mittleren und des ausseren
Ohres. Morph. Jahrbuch., Bd. 3. Tafel6-9, S. 106-151.
PLATT,JULIAB. 1891 A contribution to the morphology of thevertebrate
head, based on a study of acanthias vulgaris. Jour. Morph., vol. 5,
pls. 4 4 , pp 79-112.
SCHULTZ,
0. 1897 Grundiss der Entwicklungsgeschichte des Menschen u. der
Sliugethiere. Leipsig.
STREETER,
G. L. 1908 The peripheral nervous system in the human embryo
a t the end of the first month. Amer. Jour. Anat,., vol. 8. pls. 1-3,
pp. 2S301.
STOSS 1891 Zur Entwickelungsgeschichte des Pankreas. Anat. Anz., Bd. 6, S.
666-669.
TANDLER,
J. 1900 Zur Entwickelungsgeschichte der menschlichen Darmarterien. Anat. Hefte, Heft 71. S. 189-209.
THYNO,
F. W. 1908 Models of the pancreas in embryos of the pig, rabbit, cat
and man. Amer. Jour. Anat., vol7, pp. 489-503.
40
FRED W. THYNG
EXPLANATION OF FIGURES
ABBREVIATIONB FOR FIQ.
All.
Ao. caw
Ao. desc.
Ao. med. dor.
ATC.aor. 4
ATC.pul.
A . bas.
A . car. int.
A . coel.
A. corn. post.
A , il. com. d .
A . il. corn. s.
A. mes. inj.
A a . omp. -mes.
Ch. d .
C1.
Dien.
D. W .
EP&
F. ia.
F. iv.
F.o.
HYP.
Int.
allantois
aorta eaudalis
Ao. descendens
Ao. mediana dorsalis
arcus aorticus quartus
arcus pulmonalis
arteria basilaris
A. carotis interna
A. coeliaca
A. communicans posterior
A. iliaca communis
dextra
A. iliaca communis
sinistra
A. mesenterica inferior
Aa. omphalo-mesentericae
chorda dorsalis
cloaca
diencephalon
ductus Wolffii
epiglottis
interatrial foramen
foramen interventriculare
foramen ovale
h ypophysis
intestine
1
Islh.
M.
Mdb.
Med. sp.
Mesen.
Met.
N . sp. sec.
isthmus
mouth
processus mandibularis
medulla spinalis
mesencephalon
metanephros
nervus spinalis secundus
Neu.
Oe.
Panc. d .
Panc. v.
s.P .
st.
Telen
Th. med.
T . i.
Tr.
Urn. C .
V . hep.
V . mes. sup.
v. p .
v.pul.
V . vi.
2.
Yk.S.
a neuromere
oesophagus
pancreas dorsale
pancreas ventrale
Seesel’s pocket
stomach
telencephalon
thyreoidea mediana
tuberculum impar
trachea
umbilical cord
vena hepatica
V. mesenterica
superior
V. portae
V. pulmonalis
V. vitellina
pharyngeal diverticulum
yolk stalk
ANATOMY O F A
7.8
MM. PIG EMBRYO
41
Fig. 1 Reconstruction from a pig embryo of 7.8 mm. (Harvard Embryological
Collection 1358). The drawing illustrates especially t h e arterial system and the
epithelial portion of the digestive tract and i t s appendages. It also shows the left
atrium and ventricle of the heart, and t h e interior of the brain and spinal cord.
x .I5 diams.
42
FRED W. THYNG
ABBREVIATIONS FOR FIQ.
right fold or ridge
of aortic septum
aorta descendens
Ao. desc.
aorta ventralis
Ao. v .
arteria carotis interna
A . car. inl.
Aa. mesonephricae
A a . mes.
A. pulmonalis
A . pul.
atrium dextrum
At. d.
left fold or ridge of
b.
aortic septum
chords dorsalis
Ch. d.
ductus venoms
D. v.
ductus Wolffii
D. W .
fossa nasalis
F . nu.
medulla spinalis
Med. sp.
mesonephros
Mes .
mesencephalon
Mesen.
metanephros
Met.
metencephalon
Melen.
my elencephalon
M yelen .
Ph. P . 1 , R , 3, 4 pharyngeal pouches
a.
.
1-4
2
s. v .
sinus venosuR
thyreoidea mediana
vena cardinalis anterior
V . card. corn. d . V. cardinalis communis dextra
V . card. corn. s. V. cardinalis communis sinistra
v.card. post. V. cardinalis posterior
V . hep. corn.
V. hepatica communis
V . is.
V. intersegmentalis
V . ling.-fac.
V. linguo-facialis
V . oph.
V. ophthalmica
V. portae
v. p .
V . scard. d .
V. subcardinalis
dextra
v.scl. a.
V. subclavia dextra
V . umb. d .
V. umbilicalis dextra
ventriculus dexter
Ven. d .
T h . med.
V . card. ant.
ANATOMY OF A
7.8
MM. PIG EMBRYO
43
Fig. 2 Reconstruction from a pig embryo of 7.8 mm. (H. E. C. 1358) It represents chiefly the venous system of the right side, and the right atrium and ventricle
of the heart, and shows the relation of the aortic arches and pharyngeal pouches of
the left side. X .15 diams.
44
FRED W. THYNG
ABBREVIATIONS
Gien.
D. acus.
G. gn.
G. j u g .
G. nodos.
G. petros.
G. sl.
G. sup.
Mdb.
Med. sp.
Mesen.
Meten.
Myelen.
N . abd.
N . acc.
N . ch. t y m p .
N . fT.
diencephalon
ganglion acusticum
G. geniculatum
G. jugulare
G. nodosum
G. petrosum
G. semilunaris
G. superius
processus mandibularis
medulla spinalis
mesencephalon
metencephalon
m yelencephalon
nervus abducens
N. accessorius
N. chorda tympani
N. frontalis
POE FIG.
3
N . glos.
N . hyp.
N . lary’g. sup.
N . md.
N . mx.
N . na.
N . oc.
Nn. s p .
N . th.
N . troch.
N . tymp.
N . vag.
s. cer.
Telen.
V . aud.
V . op.
N. glossopharyngeus
N. hypoglossus
N. laryngeus superius
N. mandibularis
N. maxillaris
N. nasalis
N. oculomotorius
Nn. spinales 1-5
N. thalamicus
N. trochlearis
N. tympanicus
N. vagus
sinus cervicalis
telencephalon
vesicula auditiva
vesicula optica
ANATOMY OF A
7.8
MM, PIG EMBRYO
45
Fig. 3 Reconstruction to show the branchial arches, the exterior of the brain,
and the cranialnerves of a pig embryo of 7.8 rnrn. (H. E. C . 1358) X .15 diarns.
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