Blood islands: These irregular deeply stained masses in the vascular area are the blood island. QUESTIONS: At what level of the neural tube do you find the anteriormost somite? What is the fate of the blood islands?
The 33-hour Chick Embryo (SEE IMAGE)
Examine the 33-hour embryo under low power objective only. Never attempt to use the high power objective on the thick whole mounts to prevent breakage. To focus, lower the low power objective to a position close to the cover .slip while watching from the side then while looking through the ocular, slowly raise the objective (Please visit the main website for viewing the whole mount of 33-hour chick embryo).
The blastoderm is the living part of the chicken egg. The area pellucida is the central slipper-shaped area where the embryo lies. The region outer to this is the area opaca where ex.-embryonic blood vessels are forming and is called the area vasculosa. Note the extent of the proamnion. This is the clear area anterior to the head of the embryo. It is made up of epidermal ectoderm and endoderm layers. By 33 hours of incubation, the chick embryo now measures about 4- 5 mm in total length. Although a ventral flexion of the head is starting, it cannot go far because of the underlying yolk.
Note the 11 brain segments or neuromeres: 3 in the forebrain (telencephalon and diencephalon), 2 in the midbrain (mesencephalon), and 6 in the rhombencephalon (metencephalon and myelencephalon).
Ectodermal Derivatives
Prosencephalon: The forebrain region is the most anterior of the three primary brain vesicles. It consists of three fused neuromeres.
Anterior neuropore: This is the unfused anterior tip of the prosencephalon. It marks the region where the neural folds have not yet fused.
Optic vesicles: These are the lateral outpocketings of the prosencephalon The cavity is the opticoel.
Prosocoel: the cavity of the prosencephalon. It is continuous with the opticoel.
Infundibulum: the site of the infundibulum is marked by a ventral depression/ evagination after the appearance of the optic vesicle in the prosencephalon.
Mesencephalon: The midbrain has two fused neuromeres. The cavity is the mesocoel or the future cerebral aqueduct or the aqueduct of Sylvius.
Rhombencephalon: The hindbrain consists of six fused neuromeres. The cavity is called the rhombocoele that will later form the 4th ventricle. This brain region has too subdivisions:
Metencephalon: This brain division is caudal to the mesencephalon and is the anterior region of the hindbrain. The cavity is called the metacoel.
Myelencephalon: This is the posterior division of the rhombencephalon. The cavity is the myelocoele.
Spinal cord: This is the part of the neural tube posterior to the myelencephalon. It is differentiated from the brain parts by the absence of constrictions.
Sinus Rhomboidalis: This is the unfused posterior of the spinal cord that appears as a rhombus.
Auditory placodes: These are the forerunners of the inner ear. These are represented by ectodermal thickenings at the level of the myelencephalon.
Endodermal Derivatives
Foregut: The only region of the gut that is formed at this stage is the foregut, the anteriormost region of the digestive tube. Caudally, the foregut opens into the yolk at about the level of the first somite as the anterior intestinal portal.
Mesodermal Derivatives
Somites: These are the epimere or segmented mesoderin blocks lying beside the spinal cord .Try to count the number of somites in your specimen. Lying beside the somites are the mesomere (intermediate mesoderm or nephrotome) and the hypomere (lateral plate).
Circulatory System
Heart: The C-shaped tube lying beneath the birdbrain is the heart. Its anterior end receives the ventral aorta and its posterior end receives the omphalomesenteric veins from the area vasculosa. It is pronouncedly bent to the right. Identify the two layers making up the wall. The outer layer is the epimyocardium that will become the epicardium and the myocardium. The toner layer is the endocardium.
Now look for the following regions of the heart. The conus is the anterior arm of the U-shaped tube. Posterior to it is the ventricle, a thicker, expanded region that starts bending to the right. A larger chamber caudal to the ventricle is the atrium, which serves as the posterior arm of the U and serves as the forerunner of the auricles. Posteriorly, the atrium flattens and this venous end that receives the paired omphalomesenteric veins from the vascular area is the sinus venosus.
Ventral aorta: This single vessel is connected to the anterior or arterial end of the heart, the conus. Anteriorly, it divides into the paired ventral aortae.
Dorsal aortae: The paired dorsal aortae are formed by the ventral aortae looping dorsally in the aortic arch I and continuing posteriorly.
Vitelline arteries: These omphalomesenteric arteries arise from the dorsal aorta just caudal to the last somite. They are not distinct at this stage.
Blood islands: These irregular deeply stained masses in the vascular area are the blood island.
QUESTIONS:
- At what level of the neural tube do you find the anteriormost somite?
- What is the fate of the blood islands?
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