Photograph 1 of 45: 48 Hour Chick

Section 6 of 480 through the extraembryonic membranes (EEMs) and tip of head

At this stage in embryonic chick development three of the four extraembryonic membranes (EEMs) are visible in sections of the anterior half of the embryo. The three EEMs are the chorion (serosa), amnion, and yolk sac. (The fourth EEM, the allantois has yet to develop). 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. The space between the somatic mesoderm and splanchnic mesoderm is the extraembryonic coelom. (The extraembryonic coelom was formed by the delamination of lateral plate mesoderm into the somatic and splanchnic layers. Note that the coelom is formed in the same way in the frog.) In the center of the photograph is the third EEM, the amnion. Like the chorion, the amnion is also composed of somatopleure. Within the cavity of the amnion (amniotic cavity) a small portion of the head of the embryo is visible. The chorion (serosa) and the amnion surround the anterior half of the embryo at this stage and can be observed in the sections that follow. See Photo 35/45, Section 230 for a view of the amniotic folds. Note that the yolk sac will be visible in all the sections of the 48 hour chick.








Photograph 2 of 45: 48 Hour Chick

Section 15 of 480 through EEMs (chorion, amnion, yolk sac) and mesencephalon

In this section of the head region of the embryo, a portion of the brain, the mesencephalon is now visible. The developing brain is not linear but is bent ventrally, therefore the midbrain is sectioned before the prosencephalon (forebrain). Head mesenchyme is seen surrounding the mesencephalon.










Photograph 3 of 45: 48 Hour Chick

Section 22 of 480 through EEMs (chorion, amnion, yolk sac), metencephalon, and mesencephalon

A portion of the metencephalon is visible within the section as well as the mesencephalon.












Photograph 4 of 45: 48 Hour Chick

Section 30 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, metencephalon, and mesencephalon

Three regions of the embryonic brain are now visible in the section. These regions from top to bottom of the photograph are the myelencephalon, metencephalon, and mesencephalon. Note the characteristic thin-walled roof of the myelencephalon.





















Photograph 5 of 45: 48 Hour Chick

Section 38 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, metencephalon, and mesencephalon

Section similar in appearance to the previous section. See section 30/480, photo 4/45 for a description.

















Photograph 6 of 45: 48 Hour Chick

Section 43 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, metencephalon, mesencephalon, and diencephalon

The level of sectioning is now through the myelencephalon, the floor region of the metencephalon and mesencephalon, and the diencephalon.













Photograph 7 of 45: 48 Hour Chick

Section 52 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon and diencephalon

The myelencephalon (upper portion of photograph) and the diencephalon (lower portion of photograph) are the two portions of the embryonic brain visible at this level of sectioning. Between the two regions of the brain, a portion of the notochord is visible. Blood vessels are clearly evident. The blood vessels that are visible are the anterior cardinal veins. At this stage in development, the head region of the embryo is drained by the paired anterior cardinal veins. The tail region of the embryo is drained by the paired posterior cardinal veins. The anterior cardinal veins and the posterior cardinal veins fuse together into the common cardianl veins as they approach the heart. It is the paired common cardinal veins that empty into the sinus venosus region of the heart.










Photograph 8 of 45: 48 Hour Chick

Section 64 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, diencephalon, and eye

The level of sectioning is at the beginning of a complex region of the embryo. In this and the following six sections (Sections 67 to 91, Photos 9 to 14), a number of interesting structures are found in the vicinity of the myelencephalon (anterior cardinal veins, cranial nerve ganglia, otic capsules), the diencephalon (the developing eye, infundibulum, Rathke's pocket), and the pharynx (stomodeum, oral plate, pharyngeal cavity, pharyngeal pouches/clefts, pharyngeal arches, aortic arches, dorsal aortae).

Note the myelencephalon in the upper region of the photograph. At the base of the myelencephalon, the anterior cardinal veins are apparent. On the right hand side of the photograph (this is the embryo's left side), a cranial nerve ganglion can be seen. In the middle of the photograph, a portion of the roof of the pharynx is evident with a blood vessel (the first aortic arch) along each side. Examine the diencephalon in the lower half of the photograph. Note that the roof of the diencephalon is seen at the bottom of the section and that the floor of the diencephalon is seen near the middle of the section. The infundibulum appears as a depression or groove in the floor of the diencephalon. Above the infundibulum, a small hollow ring of cells called Rathke's pocket is visible. Rathke's pocket is an inward growing extension of ectodermal cells of the stomodeum. Together the infundibulum and Rathke's pocket will form the pituitary gland. [Recall that in the frog, the equivalent structure to Rathke's pocket was called the hypophysis]. Adjacent to the diencephalon on the embryo's left side, the double walled optic cup is readily apparent.







Photograph 9 of 45: 48 Hour Chick

Section 67 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, pharynx, diencephalon, and eye

In this more caudal section, note the myelencephalon with its characteristic thin-walled roof. Near the base of the myelencephalon, cranial nerve ganglia and the anterior cardinal veins are evident. A small portion of the pharynx is more readily apparent in this section as a cavity surrounded by endodermal cells. The first aortic arches are evident on each side of the pharynx. Note the diencephalon with the infundibulum. Between the infundibulum and the pharynx, Rathke's pocket is visible. Lateral to the diencephalon, the eyes are evident. Two components of the developing eye are now visible, the optic cups and the lens vesicles.









Photograph 10 of 45: 48 Hour Chick

Section 74 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, pharynx, diencephalon, and eye

At this level of sectioning, the structure of the pharynx has become more apparent. Note the shape of the cavity of the pharynx. Two lateral evaginations of endoderm (called the pharyngeal pouches) extend and meet with the skin ectoderm. When endoderm meets ectoderm without any intervening mesoderm, a perforation or opening usually occurs. This can be seen on the embryo's left side (right hand side of photograph) at the first pharyngeal cleft. A pair of blood vessels, the aortic arches, can be identified below the pharynx. The condensations of mesenchyme surrounding the aortic arches constitute the first pair of pharyngeal arches. Note that the term "arch" is used to describe two different structures. The pharyngeal arches are paired bars of tissue that occur anterior to and between the pharyngeal pouches. (The pharyngeal arches are also called the branchial, visceral, or gill arches.) The aortic arches are paired blood vessels that travel through the pharyngeal arches and join the ventral and dorsal aortae. At this level of sectioning the dorsal aortae can be observed between the anterior cardinal veins and the pharynx.











Photograph 11 of 45: 48 Hour Chick

Section 77 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, pharynx, diencephalon, and eye

Section similar in appearance to the previous section (Section 74/480, Photo 10/45). On each side of the myelencephalon the otic capsules are visible. Recall that the otic capsules will develop into the inner ears.












Photograph 12 of 45: 48 Hour Chick

Section 80 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, pharynx, diencephalon, and eye

The level of sectioning has reached the stomodeum which appears as a pocket in the first pharyngeal arch (mandibular arch). Note the oral plate. The oral plate is a region where ectoderm meets endoderm and at a later stage will rupture to open the mouth. This section gives an interesting view of the otic capsule and the lens vesicle on the embryo's right side. In this view we can see the "connection" of these developing structures to the exterior. Both of these structures develop from ectodermal placodes that invaginate to form vesicles that become pinched off from the overlying ectoderm. Both structures develop into parts of sense organs. From your study of the frog embryo, do you recall another sense organ that is derived from ectodermal placodes in the head region? (For an answer see Section 94/480, Photo 15/45)







Photograph 13 of 45: 48 Hour Chick

Section 87 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, notochord, pharynx, diencephalon, and eye

In this section, two separate cross sectional profiles of the embryo are evident. The more anterior portion appears below and the more posterior portion appears above. In the lower profile, as we examine the sections that follow, we will be moving towards the anterior tip of the head and brain. In the upper profile, as we examine the sections that follow, we will be moving towards the tail and will be examining sections through the myelencephalon and spinal cord, the pharynx and anterior intestinal portal, the heart, and the posterior, dorso-ventrally oriented half of the embryo.

In the upper profile, the myelencephalon and otic capsules are evident. Observe the three pairs of blood vessels that are visible: the anterior cardinal veins, the dorsal aortae, and the aortic arches. The first pharyngeal arch is also known as the mandibualr arch while the second pharyngeal arch is also known as the hyoid arch. In the lower profile, the diencephalon can be identified. Note the attachment of the optic cup to the diencephalon by the optic stalk. The optic stalk will guide the developing optic nerves from the retina (optic cup) to the brain.






Photograph 14 of 45: 48 Hour Chick

Section 91 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, notochord, pharynx, and diencephalon

In the upper cross sectional profile, the myelencephalon, anterior cardinal veins, notochord, and dorsal aortae are seen. Note the embryo's left dorsal aorta. The connection with an aortic arch is visible. Below the floor of the pharynx, the ventral aorta is evident. Blood from the heart is carried through the ventral aorta, to the aortic arches to the dorsal aortae. The dorsal aortae carry blood to the body of the embryo and also to the yolk sac by way of the vitelline arteries (Seen in Section 305/480, Photo 38/45).








Photograph 15 of 45: 48 Hour Chick

Section 94 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, notochord, pharynx, and telencephalon

In the upper cross sectional profile, observe the connection of the right aortic arch with the right dorsal aorta. In the lower cross sectional profile, sectioning has reached the most anterior portion of the brain, the telencephalon. Also seen is one of the olfactory placodes. The olfactory placodes are ectodermal thickenings that will invaginate to give rise to the nasal passages.







Photograph 16 of 45: 48 Hour Chick

Section 96 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, notochord, pharynx, telencephalon, and olfactory placode

Section similar in appearance to the previous section (Section 94/480, Photo 15/45).











Photograph 17 of 45: 48 Hour Chick

Section 99 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, notochord, pharynx, telencephalon, and olfactory placode

The level of sectioning has now reached the heart. The most anterior portion of the heart, the truncus arteriosus, is seen ventral to the pharynx. In the lower profile, both olfactory placodes can be identified on either side of the telencepahlon.









Photograph 18 of 45: 48 Hour Chick

Section 101 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, notochord, pharynx, heart, telencephalon, and olfactory placode

Identify the roof and the floor of the pharynx. Observe the groove in the floor of the pharynx. This is the thyroid rudiment. Ventral to the pharynx, paired branches of the ventral aorta (a pair of aortic arches) are visible. The structure of the truncus arteriosus is still somewhat unclear at this level.










Photograph 19 of 45: 48 Hour Chick

Section 107 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon, notochord, somites, pharynx, heart, and telencephalon

The "tube within a tube" structure of the heart is apparent in this section of the truncus arteriosus. The inner tube is the endocardium and the outer tube is the epimyocardium. In the lower profile, note the small size of the telencephalon. The level of sectioning is coming closer to the anterior tip of the brain and head.















Photograph 20 of 45: 48 Hour Chick

Section 112 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon,notochord, somites, pharynx, and heart

Beginning at the top of the section, observe the myelencephalon and the notochord. On each side of the myelencephalon, condensations of mesenchyme are apparent. The level of sectioning has reached the first pair of somites. The paired anterior cardinal veins are seen in a relatively lower location compared with previous sections. As the anterior cardinal veins approach the heart, they arch ventrally to merge with the posterior cardinal veins forming a common pair of veins, the common cardinal veins. The common cardinal veins are visible in a more caudal section (Section 129/480, Photo 23/45). Two sectional profiles of the heart are now visible. The truncus arteriosus is evident on the left hand side and the atrium is evident on the right hand side. The heart is an S-shaped tube and because of this the atrium (the third region of the heart) appears in cross section before the ventricle (the second region of the heart). Note that the atrium is located closest to the yolk sac and the truncus is closest to the chorion (serosa).










Photograph 21 of 45: 48 Hour Chick

Section 114 of 480 through EEMs (chorion, amnion, yolk sac), myelencephalon/spinal cord, notochord, somites, pharynx, and heart

The thin-walled roof of the myelencephalon has become narrower which indicates the transitional region where the myelencephalon tapers into the spinal cord. Ventral to the notochord, the paired dorsal aortae have fused to form a single, larger blood vessel, the descending aorta. On each side of the descending aorta, the anterior cardinal veins are visible. Ventral to the descending aorta, the pharynx is seen. Note that the pharynx has narrowed. The floor of the pharynx exhibits a groove called the laryngotracheal groove. The laryngotracheal groove will become separated from the pharynx as a another tube and will later develop into the larynx, the trachea, and the lungs.







Photograph 22 of 45: 48 Hour Chick

Section 122 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, pharynx, and heart

This section is similar in appearance to the previous section (Section 114/480, Photo 21/45) except that three regions of the embryonic heart are now visible. The atrium is seen immediately ventral to the pharynx with a portion of the ventricle below it. A portion of the truncus arteriosus is evident on the left hand side.











Photograph 23 of 45: 48 Hour Chick

Section 129 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, foregut, and heart

The spinal cord, somites, notochord, descending aorta, and the foregut are clearly evident in this section. Note the common cardinal veins on each side of the foregut. Three regions of the embryonic heart are visible in this section: the atrium, the ventricle, and the truncus arteriosus.











Photograph 24 of 45: 48 Hour Chick

Section 133 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, foregut, and heart

This section is similar in appearance to the previous section (Section 129/480, Photo 23/45).













Photograph 25 of 45: 48 Hour Chick

Section 140 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, foregut, and heart

Two regions of the embryonic heart, the sinus venosus and the ventricle, can be seen in this section. The sinus venosus appears ventral to the foregut and the left common cardinal vein can be seen entering it. A portion of the ventricle appears within the pericardial coelom ventral to the sinus venosus.










Photograph 26 of 45: 48 Hour Chick

Section 143 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, foregut, and heart

This section is similar in appearance to the previous section (Section 140/480, Photo 25/45). Immediately dorsal to the sinus venosus, the liver diverticulum is evident. In the chick, the liver develops as multiple, finger-like evaginations (diverticula) of the foregut.












Photograph 27 of 45: 48 Hour Chick

Section 151 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, foregut, and heart

In this section liver diverticula can be identified in both dorsal and ventral positions relative to the sinus venosus. Only a portion of the wall of the ventricle is evident.












Photograph 28 of 45: 48 Hour Chick

Section 155 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, foregut, and vitelline veins

This section is caudal to the heart and is nearing the anterior intestinal portal. Two large blood vessels, the vitelline veins, can be identified in the same relative position as the sinus venosus in previous sections. The vitelline veins carry blood from the yolk sac to the sinus venosus of the embryo.












Photograph 29 of 45: 48 Hour Chick

Section 160 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, foregut, anterior intestinal portal, and vitelline veins

This section is at the level of the anterior intestinal portal. Note the large vitelline veins on each side of the anterior intestinal portal. The extraembryonic membranes and cavities are labelled in this section as a review.












Photograph 30 of 45: 48 Hour Chick

Section 167 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, anterior intestinal portal, and vitelline veins

This section is similar in appearance to the previous section (Section 160/480, Photo 29/45).












Photograph 31 of 45: 48 Hour Chick

Section 174 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, and vitelline veins

Observe the spinal cord, somites, notochord, and descending aorta. Two small blood vessels, the posterior cardinal veins, are visible on each side of the descending aorta. The posterior cardinal veins carry blood from within the posterior half of the embryo to the heart.












Photograph 32 of 45: 48 Hour Chick

Section 187 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, and vitelline veins

This section is similar in appearance to the previous section (Section 174/480, Photo 31/45).














Photograph 33 of 45: 48 Hour Chick

Section 205 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, and nephrotome

In this section note that we have reached the part of the embryo where torsion has not occurred. This caudal portion of the embryo is laying dorso-ventrally with respect to the yolk. Observe the descending aorta and the posterior cardinal veins on each side. Note the condensation of mesenchyme (arising from intermediate mesoderm) lateral to the descending aorta and ventral to the posterior cardinal veins. This is the embryonic kidney rudiment or nephrotome.













Photograph 34 of 45: 48 Hour Chick

Section 217 of 480 through EEMs (chorion, amnion, yolk sac), spinal cord, notochord, somites, and nephrotome

This section is similar in appearance to the previous section (Section 205/480, Photo 33/45) except that the descending aorta has divided into the paired dorsal aortae.













Photograph 35 of 45: 48 Hour Chick

Section 230 of 480 through amniotic folds, yolk sac, spinal cord, notochord, somites, and nephrotome

In this section, note the amniotic folds. The amniotic folds are folds of somatopleure. At 48 hours of incubation, the development of the amnion and chorion (serosa) has not yet been completed.

In the following sections (Sections 250 to 400, Photos 36 to 41), note the reduction in the amniotic folds and the flattening of the embryo. Observe the structural continuity and diminishing size of the spinal cord, notochord, and dorsal aortae. In Section 305/480, Photo 38/45 the branching of the dorsal aortae into the vitelline arteries can be identified. In the regions of the nephrotomes, note the presence of a pair of small ducts, the nephric ducts (pronephric ducts, mesonephric ducts, Wolffian ducts).








Photograph 36 of 45: 48 Hour Chick

Section 250 of 480 through amniotic folds, yolk sac, spinal cord, notochord, somites, and nephrotome
















Photograph 37 of 45: 48 Hour Chick

Section 280 of 480 through yolk sac, spinal cord, notochord, somites, and nephrotome
















Photograph 38 of 45: 48 Hour Chick

Section 305 of 480 through yolk sac, spinal cord, notochord, somites, nephric ducts, and vitelline arteries


















Photograph 39 of 45: 48 Hour Chick

Section 315 of 480 through yolk sac, spinal cord, notochord, somites, and nephric ducts


















Photograph 40 of 45: 48 Hour Chick

Section 347 of 480 through yolk sac, spinal cord, notochord, somites, and nephric ducts


















Photograph 41 of 45: 48 Hour Chick

Section 400 of 480 through yolk sac, spinal cord, notochord, and somites


















Photograph 42 of 45: 48 Hour Chick

Section 425 of 480 through yolk sac, spinal cord, and segmental mesoderm

At this posterior level of the embryo, the spinal cord and notochord are no longer distinct and the somites have given way to the less differentiated, segmental mesoderm. Sectioning is approaching the tailbud.
















Photograph 43 of 45: 48 Hour Chick

Section 439 of 480 through yolk sac, tail bud, hindgut, and posterior intestinal portal

This section is at the level of the tailbud and hindgut. Observe the posterior intestinal portal.















Photograph 44 of 45: 48 Hour Chick

Section 447 of 480 through yolk sac, tail bud, and hindgut

The level of sectioning is through the hindgut and posterior portion of the tailbud. Sectioning is reaching the end of the embryo.
















Photograph 45 of 45: 48 Hour Chick

Section 460 0f 480 through extraembryonic germ layers

This section is through the extraembryonic germ layers that are posterior to the embryo. The somatopleure (a double layer made up of a layer of ectoderm and a layer of somatic mesoderm) is seen on the left hand side of the coelom. The somatopleure will give rise to the tail fold of the amnion. Recall that the outer layer of the folded somatopleure gives rise to the chorion (serosa) and that the inner layer of the folded somatopleure gives rise to the amnion. On the right hand side of the coelom, the splanchnopleure (a double layer made up of a layer of splanchnic mesoderm and a layer of endoderm) is visible. This is the yolk sac which covers a much larger area of the yolk than the embryo. Many more sections would be required to reach the outer edge of the yolk sac. This is the last photograph in the series.

THE END OF THIS CHICK EMBRYO