24-hour chick embryo

24 hour chick embryo

through head fold and subcephalic spaceIn this more caudal section, the continuity of the neural folds are evident. The outer layer of the head fold (also called the head process) is composed of ectoderm. Below the head fold note the subcephalic space, a layer of ectoderm, and a layer of endoderm.

through head fold, foregut, and notochordAt this level, sectioning has now reached the notochord. The notochord is evident ventral to the neural folds. Ventral to the notochord, the foregut appears as a smile-shaped cavity delimited by thin endodermally derived walls. Note that the mid portion of the floor of the foregut is slightly thickened. Below this region is a region of slightly thickened ectoderm. This region is the oral plate which will become perforated at a later date to form the mouth. Below the head fold is the subcephalic space and extraembryonic germ layers. Note the proamnion (ectoderm and endoderm) below the head fold. Lateral to the proamnion, layers of mesoderm are visible between the ectoderm and endoderm. The cavity that is evident between the layers of mesoderm is the coelom.

through anterior intestinal portalIn this section, note that the neural folds have come together to form the neural tube. This section is also at the level of the open foregut. Note that the endoderm of the foregut is continuous with the rest of the endoderm. The opening of the closed foregut is called the anterior intestinal portal. Examine the thickened, splanchnic mesoderm on each side of the anterior intestinal portal. This region of thickened, splanchnic mesoderm is prospective cardiac mesoderm.

through somitesIn this section through a pair of somites, the neural folds have not yet closed together. The somites developed from dorsal mesoderm (D) and are located on each side of the neural folds and notochord. Lateral and adjacent to the somites is a small region of mesoderm known as the intermediate mesoderm (I). The intermediate mesoderm will develop into the kidneys. Lateral to the intermediate mesoderm, is the lateral plate mesoderm (L) which will delaminate to form the somatic and splanchnic mesodermal layers.

through anterior tip of head foldIn this section, the anterior tip of the head fold can be seen. The portions that are visible at this level are portions of the neural folds. Recall that the neural folds form from a thickened region of dorsal ectoderm by a process called neurulation. The neural folds will fuse to form the neural tube, the forerunner of the brain and spinal cord. Below the neural folds, a space is evident. This space is called the subcephalic space or pocket. Below the subcephalic space is a region consisting of a layer of ectoderm and a layer of endoderm. This region is called the proamnion.

through region of Hensen's nodeThe level of sectioning has now reached Hensen's node, the cephalic end of the primitive streak. Hensen's node is the functional equivalent to the dorsal lip of the blastopore in the amphibian embryo.

through primitive streakIn the centre of the photograph, note the primitive streak which consists of the primitive groove and the two primitive ridges. Note the ectoderm, mesoderm, and endoderm. This photograph is the last in the series. Note that there are 35 more sections to reach THE CAUDAL END OF THIS EMBRYO.

33 HOUR CHICK EMBRYO

through head at level of prosencephalon and optic vesiclesIn this more caudal section, the optic vesicles are seen as lateral bulges of the prosencephalon. The optic vesicles are the forerunners of the retinal portions of the eyes. Note the close proximity of the optic vesicles to the overlying head ectoderm. The optic vesicles will induce this overlying ectoderm to thicken, invaginate, and form the lens vesicles.

The extraembryonic germ layers are evident below the developing head. Note that the mesoderm does not extend into the region below the head fold. This region, known as the proamnion, consists of a layer of ectoderm underlain with a layer of endoderm. The lack of mesoderm at this stage makes this region very pale staining in whole mounts. The proamnion is a region that will eventually be overgrown and disappear.

through head at level of mesencephalon, notochord, foregut, and oral plateAt this level, sectioning has now reached the notochord. The notochord is evident ventral to the mesencephalon (midbrain). Ventral to the notochord, the foregut appears as a smile-shaped cavity delimited by thin endodermally derived walls. Note that the mid portion of the floor of the foregut is slightly thickened. Below this region is a region of slightly thickened ectoderm. This region is the oral plate which will become perforated at a later date to form the mouth. Below the head fold is the subcephalic space and extraembryonic germ layers.

through rhombencephalon, foregut, and heartThis section is posterior to the head fold. Note the absence of the subcephalic space. The portions of the embryo that will become part of the body of the embryo are continuous with the portions that will form the extraembryonic membranes. Note the rhombencephalon (hindbrain), notochord, and foregut. Ventral to the foregut, observe the anterior portion of the developing heart.

through rhombencephalon, foregut, and vitelline veinsSectioning has now reached the posterior portion of the heart where the heart bifurcates into the vitelline veins. The vitelline veins (= omphalomesenteric veins) bring blood from the yolk sac to the posterior portion of the heart.

through rhombencephalon, anterior intestinal portal, and vitelline veinsThis section is posterior to the heart at the level of the anterior intestinal portal. Note that the endoderm of the foregut is continuous with the extraembryonic endoderm. The vitelline veins are located on each side of the anterior intestinal portal.

through spinal cord and fourth pair of somitesRecall that somites are discrete blocks of tissue arising from dorsal mesoderm. The somites will develop into the vertebrae and body muscles, and will contribute cells to the dermis of the skin.

through neural foldsNote the flat appearance of the embryo in this section. At this level, the formation of the neural tube has not been completed and the neural folds are evident. Note the notochord and the layers of ectoderm, mesoderm, and endoderm.

through Hensen's nodeThe level of sectioning has now reached Hensen's node, the cephalic (=anterior) end of the primitive streak. Hensen's node is homologous to the dorsal lip of the blastopore in the amphibian embryo. Review the significance of these structures.

through primitive streakIn the centre of the photograph, the primitive streak which consists of the primitive groove bordered on each side by the primitive ridges is visible. Note the ectoderm, mesoderm, and endoderm. This photograph is the last in the series. Sixty-two more sections were required to reach THE CAUDAL END OF THIS EMBRYO.