Section 15 of 380 through extraembryonic membranes (EEMs)
At this stage in embryonic chick development three of the four extraembryonic
membranes (EEMs) are visible in all sections of the 72 hour chick.
In this section, three EEMs are visible and these are the chorion (=serosa),
amnion, and yolk sac. The yolk sac can be distinquished from the other
two EEMs by the presence of blood vessels. The yolk sac lies on the right
side of the photograph and is composed of splanchnopleure which is a thin
layer of endoderm (closest to the yolk) and a thin layer of splanchnic
mesoderm. It is from the splanchnic mesoderm that the blood vessels
developed. The EEM on the left hand side of the photograph is the chorion
(serosa). The chorion is composed of somatopleure which is a layer of ectoderm
and a layer of somatic mesoderm. In the center of the photograph is a portion
of the third EEM, the amnion. Like the chorion, the amnion is also
composed of somatopleure. The fourth EEM, the allantois has just begun
to develop as an evagination of the hindgut and can be observed in the
posterior of the embryo (Section 325/380, Photo 39/45).
Section 25 of 380 through EEMs (chorion, amnion, yolk sac) and myelencephalon
In this section of the head region of the embryo, a portion of the hindbrain,
the myelencephalon is now visible. This section is through the roof of
the myelencephalon. Note that the walls of the roof of the myelencephalon
are very thin.
Section 29 of 380 through EEMs (chorion, amnion, yolk sac) and myelencephalon
Sectioning of the myelencephalon has now reached the thicker appearing
lateral walls (upper portion of photograph). In the lower part of the section,
the level of sectioning is still through the thin-walled roof of the myelencphalon.
Section 36 of 380 through EEMs (chorion, amnion, yolk sac) and myelencephalon
At this level of sectioning, both the lateral walls (upper portion of section)
and the roof (lower portion of photograph) are visible. Note that the upper
portion of the myelencephalon appears narrower. This indicates a region
where the myelencephalon is tapering into the spinal cord. The beginnings
of the first pair of somites are evident on each side of the narrowed region
of the myelencephalon.
Section 49 of 380 through EEMs (chorion, amnion, yolk sac), spinal cord,
otic capsules, and myelencephalon
Both the spinal cord and myelencephalon are visible in this section. Somites
are evident on each side of the spinal cord. The otic capsules appear on
each side of the myelencephalon. Note the scalloped appearance of the walls
of the myelencphalon. Recall that at 33 hours of incubation, the hindbrain
of the chick showed marked segmentation into neuromeres. At the 72 hour
stage, the neuromeres are evident in cross section as scallops in the wall
of the myelencephalon.
Section 56 of 380 through EEMs, spinal cord, somites, notochord, otic capsules,
myelencephalon, and metencephalon
The spinal cord, myelencephalon, and metencephalon are visible in this
section. Between the spinal cord and the myelencephalon two portions of
the notochord are evident. Like the brain, the notochord curves ventrally,
and hence two portions are evident in the section. Also apparent are structures
that occur in pairs. These are the somites, the anterior cardinal veins,
and the otic capsules.
Section 61 of 380 through EEMs, spinal cord, somites, notochord, myelencephalon,
and metencephalon
At this level, the spinal cord, myelencephalon, and metencephalon are still
visible. In the region between the spinal cord and hindbrain, note that
the notochord appears twice. The pair of dorsal aortae are seen merging
into one blood vessel called the descending aorta. A number of profiles
of the anterior cardinal veins are also visible.
Section 65 of 380 through EEMs, spinal cord, somites, notochord, pharynx,
and metencephalon
The level of sectioning has now reached the beginning of the pharygeal
region. The cavity of the pharynx is visible as well as a pair of pharyngeal
pouches and pharyngeal arches. Note the orientation of the 72 hour chick.
The embryo has both a cranial flexure and a cervical flexure and serial
cross sectioning of the pharynx begins at its roof and proceeds through
to the pharyngeal floor. In the photograph, the descending aorta and a
pair of anterior cardinal veins are still visible ventral to the spinal
cord and notochord. On each side of the metencephalon, note the cranial
nerve ganglia.
Section 70 of 380 through EEMs, spinal cord, somites, notochord, pharynx,
metencephalon, and roof of mesencephalon
In this more caudal section, note the pharynx with pharyngeal pouches.
Blood vessels called the aortic arches run through the pharyngeal arches.
Note that the term "arch" is used to describe two different types of structures.
It is used to describe a bar of tissue as well as a blood vessel. In the
upper region of the section, observe the spinal cord, notochord, descending
aorta, and the pair of anterior cardinal veins. Just below the pharynx,
a small pair of blood vessels called the internal carotid arteries can
be seen. The internal carotid arteries are anterior extensions of the dorsal
aortae that carry blood to the brain. In the lower regions of the section,
examine the notochord, metencephalon, and the beginnings of the mesencephalon.
Section 72 of 380 through EEMs, spinal cord, somites, notochord, pharynx,
metencephalon, and mesencephalon
Section similar in appearance to the previous section. See section 70/380,
photo 9/45 for a description.
Section 79 of 380 through EEMs, spinal cord, somites, notochord, pharynx,
thyroid rudiment, stomodeum, metencephalon, and mesencephalon
Sectioning has now reached the floor of the pharynx. The aortic arches
are visible in the second and third pharyngeal arches. Observe the small
circular cavity in the floor of the pharynx between the second pharyngeal
arches. This depression in the floor of the pharynx is the rudiment of
the thyroid gland. Note the cavity that appears below the thyroid rudiment.
This cavity is the stomodeum. Immediately below the stomodeum, the small
internal carotid arteries can still be observed. In the upper portion of
the pharyngeal region, note the narrowing of the pharyngeal cavity.
Section 82 of 380 through EEMs, spinal cord, somites, notochord, pharynx,
laryngotracheal groove, thyroid rudiment, stomodeum, and mesencephalon
In this more caudal section, the floor of the thyroid rudiment is now evident.
The opening in the floor of the pharynx that surrounds the thyroid rudiment
is the ventral aorta where the aortic arches begin. Above the ventral aorta,
note that the pharynx has narrowed. The floor of the pharynx in this region
has a depression called the laryngotracheal groove. At a later stage in
development, the laryngotracheal groove will separate from the pharynx
and will develop into the larynx, trachea, and lungs. Observe the coelomic
cavities on each side of the pharynx and laryngotracheal groove.
Review the blood vessels that are visible at this level of sectioning.
The blood vessels that are evident in this section are the ventral aorta,
aortic arches, descending aorta, and the anterior cardinal veins.
Section 88 of 380 through EEMs, spinal cord, somites, notochord, pharynx,
trachea, heart, stomodeum, and mesencephalon
In the upper part of the section, below the narrowed pharyngeal cavity, note that
the laryngotracheal groove has expanded into the trachea. Caudally, the trachea
will divide into two branches (the lung buds). Note the common cardinal veins
on each side of the intraembryonic coelom.
At this level, sectioning has reached the heart forming region
of the embryo. In the center of the photograph, examine the ventral aorta
and sinus venosus. By 72 hours of incubation, the embryonic heart tube
has become twisted upon itself and forms a simple loop. Therefore, in a
cross sectional series, the most anterior and posterior portions of the
heart appear simultaneously in the early sections. Ventral and lateral
to the ventral aorta, small portions of the second pharyngeal arch, the
hyoid arch, are evident. The mandibular and the maxillary processes of
the first pharyngeal arch can be clearly distinguished. In the lower part
of the section, observe the anterior cardinal veins and the mesencephalon.
Section 93 of 380 through EEMs, spinal cord, somites, notochord, esophagus,
lung buds, heart, infundibulum, Rathke's pocket and mesencephalon
In the upper part of the section, observe the spinal cord, notochord, somites,
descending aorta, and common cardinal veins. Ventral to the descending
aorta, the esophagus and lung buds are evident. Portions of the sinus venosus,
atrium, and truncus arteriosus are visible in the center of the photograph.
In the lower half of the photograph, sectioning has reached the level of
the infundibulum and Rathke's pocket.
Section 99 of 380 through EEMs, spinal cord, somites, notochord, esophagus,
heart, diencephalon, and mesencephalon
Section similar in appearance to previous section (Section 93/380, Photo
14/45) except that sectioning has passed the level of the infundibulum
and has reached the level of the diencephalon.
Section 105 of 380 through EEMs, spinal cord, somites, notochord, stomach,
heart, diencephalon, eye, and mesencephalon
This section is caudal to the pharynx/esophagus and the lung buds and is
at the level of the stomach region of the digestive tract. Three regions
of the embryonic heart are visible, these being the sinus venosus, atrium,
and the truncus arteriosus. (The fourth region, the ventricle, is posterior
to this section). Lateral to the diencephalon, portions of the eye are
evident. Note the optic cups on each side and the lens on the left (the
embryo's right side).
Section 108 of 380 through EEMs, spinal cord, somites, notochord, stomach,
heart, diencephalon, eye, and mesencephalon
Section similar in appearance to the previous section (Section 105/380,
Photo 16/45) except that in this section the connecting stalks between
the optic cups and the diencephalon are visible. The optic stalks will
become the optic nerves in the adult.
Section 115 of 380 through EEMs, spinal cord, somites, notochord, stomach,
heart, diencephalon, eye, and mesencephalon
Beginning at the top of the section, locate the spinal cord, notochord,
somites, and descending aorta. On each side of the descending aorta, observe
the posterior cardinal veins. Below the descending aorta, examine the intraembryonic
coelom and the stomach region of the digestive tract. In the center of
the section, note the sinus venosus, atrium, and truncus arteriosus. In
the lower half of the section, observe the diencephalon, optic cups, lenses,
and the mesencephalon.
Section 118 of 380 through EEMs, spinal cord, somites, notochord, stomach,
heart, diencephalon, eye, and mesencephalon
Section similar in appearance to previous section (Section 115/380, Photo
18/45).
Section 122 of 380 through EEMs, spinal cord, somites, notochord, stomach,
heart, diencephalon, and eye
In this more caudal section, examine the region of the digestive tract
and sinus venosus. In this section, the transition of the digestive tract
from the region of the stomach to the region of the duodenum is marked
by the presence of portion of the liver diverticulum. Ventral to the sinus
venosus, note the continuity of the atrium and the ventricle.
Section 129 of 380 through EEMs, spinal cord, somites, notochord, kidney
rudiments, duodenum, vitelline veins, olfactory pits, telencephalon,
and diencephalon
The spinal cord, notochord, somites, descending aorta, and posterior cardinal
veins are evident in the upper portion of the section. Ventral to the posterior
cardinal veins, note the rudiments of the first embryonic kidney, the pronephros.
In the chick, the development of the pronephros remains rudimentary and
at 72 hours has already begun to degenerate. In more caudal sections, the
second embryonic kidney, the mesonephros, will become evident. In the center
of the section, note the duodenum, vitelline veins, liver diverticula,
and ventricle. In the bottom of the section, note the diencephalon and
the lateral expansions that mark the beginning of the telencephalon. Also
note the olfactory pits.
Section 132 of 380 through EEMs, spinal cord, somites, notochord, kidney
rudiments, duodenum, vitelline veins, olfactory pits, telencephalon,
and diencephalon
Section similar in appearance to the previous section (Section 129/380,
Photo 21/45).
Section 136 of 380 through EEMs, spinal cord, somites, notochord, kidney
rudiments, duodenum, vitelline veins, telencephalon, and diencephalon
Section similar in appearance to previous sections (See Section 129/38,
Photo 21/45 for description) except that sectioning has passed the olfactory
pits.
Section 145 of 380 through EEMs, spinal cord, somites, notochord, kidney
rudiments, duodenum, vitelline veins, olfactory pits, telencephalon,
and diencephalon
In the upper half of the photograph, note the spinal cord, notochord, somites,
and descending aorta. On each side of the descending aorta, the posterior
cardinal veins and pronephroi (plural of pronephros) are evident. Lateral
to the posterior cardinal veins and pronephroi, observe the bulges in the
lateral body walls that mark the beginning of the wing buds. In the center
of the photograph, the duodenum, vitelline veins, and ventricle are still
evident. In the bottom of the photograph, note the constriction between
the telecephalon and diencephalon.
Section 152 of 380 through EEMs, spinal cord, somites, notochord, wing
bud, kidney rudiments, duodenum, anterior intestinal portal, vitelline
veins, olfactory pits, and telencephalon
In the upper half of the section, the spinal cord, wing buds, and descending
aorta are clearly evident. On each side of the descending aorta, the posterior
cardinal veins and the pronephroi are seen.
In the center of the section, note the duodenum and the anterior
intestinal portal. On each side of the gut, the vitelline veins are evident.
Recall that the vitelline veins carry blood from the yolk sac to the most
posterior region of the heart, the sinus venosus. To the left of the anterior
intestinal portal, a portion of the wall of the ventricle is evident. In
the bottom of the photograph, the telencephalon can be seen.
Section 158 of 380 through EEMs, spinal cord, somites, notochord, wing
bud, kidney rudiments, anterior intestinal portal, vitelline veins,
and tip of head
Section similar in appearance to the previous section (Section 152/380,
Photo 25/45) except that the section is caudal to all parts of the brain
and heart. Only the most anterior tip of the head is still evident.
Section 170 of 380 through EEMs, spinal cord, somites, notochord, wing
bud, kidney rudiments, and midgut
This section is caudal to the head, heart, and the anterior intestinal
portal. The section is at the level of the open midgut. The kidney
rudiments are present as the second embryonic kidney, the mesonephros.
In the chick, the development of the pronephros remains rudimentary
and at 72 hours has already begun to degenerate. The transition from pronephros
to mesonephros is a morphologically indistinct zone located near the level
of the anterior intestinal portal.
Section 190 of 380 through EEMs, spinal cord, somites, notochord, wing
buds, kidney rudiments, and midgut
Section similar in appearance to the previous section (Section 170/380,
Photo 27/45).
Section 198 of 380 through EEMs, spinal cord, somites, notochord, wing
buds, kidney rudiments, and midgut
Note that this section (Section 198/380, Photo 29/45) and the following
sections (Sections 212 to 360, Photos 30 to 45) were photographed at a
higher magnification than the preceeding sections (Sections 15 to 190,
Photos 1 to 28).
Three of the four extraembryonic membranes are readily identified
at this magnification. Note the chorion, amnion, and yolk sac.
Observe the spinal cord and notochord. At this magnification
it is possible to distinguish different regions within the somite. The
portion of the somite that is closest to the spinal cord and notochord
is called the sclerotome. At this stage the sclerotome consists of moderately,
densely packed cells that occupy the largest area of the somite. The sclerotomes
will develop into the vertebrae. Adjacent to the sclerotome is a thin region
called the myotome. The myotome will develop into the muscles of the back
and body walls. Adjacent to the myotome and occupying the outermost region
of the somite is the dermatome. The dermatome will contribute cells to
the development of the dermis of the skin.
Ventral to the notochord, the descending aorta is evident. On
each side of the descending aorta the rudiments of the second embryonic
kidney (the mesonephros) are visible. Dorsal to the mesonephroi, the posterior
cardinal veins are evident. Note that at this level of sectioning the embryo's
left wing bud is clearly apparent.
Section 212 of 380 through EEMs, spinal cord, somites, notochord,
wing buds, kidney rudiments, and midgut
In this more caudal section, the rudiments of the mesonephric kidney are
more clearly seen. In the condensed knot of the mesonephrogenic mesenchyme
(=nephrotome), a mesonephric tubule is evident. Lateral to the mesonephric
tubule, the mesonephric duct can be clearly seen.
Note that the descending aorta appears wider than its height
with a small "partition" from its dorsal wall. In the sections that follow
the descending aorta will be seen to divide into two blood vessels, the
paired dorsal aortae.
Section 219 of 380 through EEMs, spinal cord, somites, notochord,
kidney rudiments, and midgut
This section is similar in appearance to the previous section (Section
212/380, Photo 30/45) except that the descending aorta has divided into
the paired dorsal aortae.
Section 224 of 380 through EEMs, spinal
cord, somites, notochord, kidney rudiments, midgut, and vitelline
arteries
This more caudal section is at the level of the vitelline arteries (=omphalomesenteric
arteries). The vitelline arteries are large branches of the dorsal aortae
that carry blood out of the embryo to the blood vessel network of the yolk
sac. Note that the dorsal aorta of the embryo's left side is continuous
with a blood vessel, the vitelline artery, that travels out to the yolk
sac. Find and observe the right vitelline artery.
Section 240 of 380 through EEMs, spinal cord, somites, notochord,
kidney rudiments, and midgut
This section is caudal to the vitelline arteries. Note the smaller diameter
of the dorsal aortae. At this level of sectioning the spinal cord,
notochord, somites, posterior cardinal veins, kidney rudiments, and the
midgut are readily evident.
Section 265 of 380 through EEMs, spinal cord, somites, notochord,
leg buds, kidney rudiments, hindgut, and posterior intestinal portal
The study of the serial cross sections of the 72 hour chick has now reached
the posterior quadrant of the embryo. Sectioning is caudal to the midgut
and has reached the region of the hindgut where it opens at the posterior
intestinal portal. At this level, note the thickenings of the lateral body
walls that indicate the beginning of the leg buds.
Section 285 of 380 through EEMs, spinal cord, somites, notochord,
leg buds, kidney rudiments, hindgut, and posterior intestinal portal
Section similar in appearance to the previous section (Section 265/380,
Photo 34/45).
Section 295 of 380 through EEMs, spinal cord, somites, notochord,
leg buds, kidney rudiments, hindgut, and posterior intestinal portal
Section similar in appearance to previous sections. See Section 265/380,
Photo 34/45 for a description.
Section 305 of 380 through EEMs, spinal cord, somites, notochord,
kidney rudiments, and hindgut
Sectioning has now reached the level of the floored-in hindgut which is
posterior to the posterior intestinal portal. In the chick, the hindgut
is formed as the tail fold passes under the posterior trunk region. The
spinal cord, notochord, somites, and dorsal aortae are very distinct. Notice
that the mesonephric ducts have become smaller in diameter and that the
posterior cardinal veins are no longer identifiable.
Section 312 of 380 through EEMs including allantois, spinal cord,
somites, notochord, kidney rudiments, and hindgut
Sectioning has reached the level of the allantois. The allantois is the
fourth extraembryonic membrane that develops as an evagination of the hindgut.
Like the yolk sac it is composed of a layer of endoderm and a layer of
splanchnic mesoderm.
Section 325 of 380 through EEMs including allantois, spinal cord,
somites, notochord, kidney rudiments, and hindgut
The continuity of the allantois with the hindgut is evident in this section.
Review the location of the three other EEMs; the chorion, amnion, and yolk
sac. At this level of sectioning, it is possible to distinguish the two
layers of the somatopleure of the amnion. Note that the coelom within the
embryo (intraembryonic coelom) is separated from the coelom outside of
the embryo (extraembryonic coelom) by the ventral body wall.
Section 333 of 380 through EEMs including allantois, spinal cord,
somites, notochord, kidney rudiments, and hindgut
Section similar in appearance to the previous section (Section 325/380,
Photo 39/45).
Section 338 of 380 through EEMs , spinal cord, somites, notochord,
kidney rudiments, and cloaca
Sectioning has reached the cloaca, the most posterior portion of the digestive
tract. In the region of the cloacal membrane where endoderm meets ectoderm,
an opening will later form.
Section 340 of 380 through EEMs , spinal cord, somites, notochord,
kidney rudiments, and cloaca
Section similar in appearance to previous section (Section 338/380, Photo
41/45).
Section 348 of 380 through EEMs , spinal cord, somites, notochord,
and cloaca
Sectioning has reached the curved portion of the tail. Two pairs
of somites are seen on each side of the spinal cord. The notochord and
a small portion of the cloaca are also evident.
Section 353 of 380 through EEMs , and spinal cord
In this section, the spinal cord is grazed obliquely with small portions
of somites and undifferentiated mesoderm on each side.
Section 360 of 380 through EEMs , and spinal cord
In this last photograph of the series, the spinal cord and a small amount
of mesoderm surrounded by ectoderm is seen. The amnion, chorion, and the
yolk sac are also evident.
THE END OF THE SERIAL CROSS SECTIONS OF THE 72 HOUR CHICK