24 Hour Chick Embryo Whole Mount
surrounding the head; contains only endoderm and ectoderm
Area opaca -
Proamnion Region anterior to the developing avian embryo,
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It will later become invaded with mesoderm and become incorporated into the amnion
Peripheral region of the chick blastodisc surrounding the area pellucida and in direct contact with the yolk
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Peripheral region of the blastoderm where the cells are continuous with the underlying yolk that r enders the area opaque In early development , it is diff erentiated into three zones: 1)
A peripheral zone, known as the margin of growth, where the cells have proliferated over the y olk
Neural folds -
mid-dorsal region of the developing embryo Neural groove -
A zone junction, an intermediate region where the cells cut free from the yolk and are added to the area pellucida
3)
A median longitudinal groove which is formed by the rolling up the neural plate
without becoming adherent to it 2)
Margins of the neural late that are about to fuse at the
Head fold Folded region of the blastoderm, represented by a da rk
An inner zone, known as the germ wall, whose cells
crescentic line that marks the boundary of the heads of
originated from the zone junction, thus containing
the embryo
yolk granules. This region marks the transition from the area opaca to the area pellucida. T his area develops into two regions: a)
Notochord Longitudinal bands of cells extending caudal from the regional of the neural folds to the Hensen’s node
Area opaca vasculosa – inner region of the area opaca where thickenings of the splanchnic mesoderm for bloodi isalnds
b)
outer region of the area Area opaca vitelline – outer
Foregut First part of the digestive tract with a definite floor composed of the endodermal cells.
opaca which initially lacks any blood islands or blood vessels
Central region of the blastodisc where the primitive streak
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Appears like prominent arched line beneath the neural folds
forms -
Appears as a dome-shaped structure
Anterior Intestinal portal Opening of the foregut in developing amniote embryos
Area pellucida -
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Lies directly over the subgerminal cavity and under the
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Actually continuous with the lateral walls of the foregut
microscope, appears translucent
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Ovwed to the thinness of the blastoderm which consist of
As the development progresses and the gut elongates, the opening moves posteriorly and ultimately becomes part of
an outer layer of epiblast, from which ectoderm and
the yolk stalk
mesoderm will later differentiate, and an inner layer of Mesenchyme of the head Lose network of cells occupying the space between the
hypoblast
foregut and the ectoderm of thead
Primitive streak -
Longtidunal cleft formed on surface of amniote blastodisc
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by a convergence of cells Gastrulation begins as epiblast cells migrate towards and
Mesoderm o
Faint line extending across the area pellucida at the level of the anterior end of the notochord
into the streak o
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Anterior border of the mesoderm
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Primitive groove
Thickened splanchnic mesoderm
Central furrow of the thickening
Darker line each of the lateral border of the
Primitive ridges
anterior intestinal portal. Formed by the
Margins of the primitive streak flanking the
delamination of the lateral plate mesoderm.
primitive groove on either side
Later forms the heart, part of circulatory system
Primitive pit
and many other structures.
Indented region of the blastoderm at the anterior end of the primitive streak o
Hensen’s node
Somite -
can be observed
Thickened area at the anterior end of the primitive streak through which cells invaginate to form the notochord. It is homologous to the dorsal lip of the blastopore in amphibian embryos. Syn: primitive knot
Apparent division of the dorsal mesoderm into somites
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Three of four pairs of somites maybe seen lying immediately lateral to the neural folds
Unsegmented mesoderm Undivided region of the mesoderm posterior to the
Splanchnopleure -
somites
Cellular layer consisting of the splanchnic mesoderm and endoderm Form: part of the body wall in the embryo as well as the yolk sac and allantois
Transverse Section
Coelom -
Section through the pharyngeal membrane Head ectoderm Layer of cuboidal cells enclosing the head Cells are cuboidal in origin
Subgerminal cavity The space beneath the hypoblast within the area pellucida Form: midgut
Neural folds Margins of the neural epithelium that have yet fused at the midline Anterior neuropore -
Temporary external opening of the neurocoel
Neural groove -
A median longitudinal groove which
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formed by the rolling up of the neural plate
The body cavity formed when the somatic mesoderm and the mesoderm separates from each other
Section through the anterior intestinal portal In 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. Examine the thickened, splanchnic mesoderm on each side of the anterior intestinal portal. This region of thickened, splanchnic mesoderm is prospective cardiac mesoderm. Neural tube
Prechordal plate a mass of chordamesodermal cells located anterior to the
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A tube of tissue
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Formed by a thickening and rolling up of the neural plate
notochord which will form head mesenchyme
during embryonic neurulation Form: brain and spinal cord of the animal
Notochord -
a small medial mass of cells between the foregut and the neural plate
Neural crest -
lying above the neural tube after its formation
Foregut -
a dorsoventral flattened tube with endodermal cells
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located beneath neural plate
Oral plate -
region where the floor of the foregut is in contact with the neural ectoderm of the head
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In lateral development this structure will break open to form the mouth Syn: oropharyngeal membrane
Head fold Marked by the downward bend of the membranes around the head region of the developing embryo Anterior intestinal portal -
Opening of the foregut in developing amniote embryos
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As development progresses and the gut elongates, the opening moves posteriorly and ultimately becomes part of
Mesenchyme -
Migrate throughout the embryo to Form: ganglia, pigment cells, part of gill arches, etc.
making up its wall -
A group of cells found at the edges of the neural plate and
Loosely scattered cells filling up the spaces between the foregut and the neural plate
Proamnion Region of the blastoderm beneath the embryo’s head Composed of the ectoderm and endoderm layer
the yolk stalk. Amnio-cardiac vesicle -
The region of the coelom that will
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Form: amnion and the heart Thickened splanchnic mesoderm making up the proximal wall of the semi-cardiac vesicle is the forerunner of the heart
Subcephalic pocket -
The cavity formed beneath the embryonic head as the head fold develops
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On the hand, the dorsal wall of the vesicle, composed of the somatopleure, is involved in the formation of the amnion.
Somatopleure -
The membrane composed of the ectoderm and somatic mesoderm Forms: part of body wall as well as the chorion and amnion
Section through the midgut
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Primitive pit
The middle part of the gut that has the yolk for the floor of its cavity
A depression, slightly deeper than the primitiv e
is called the midgut. In this section, find the endoderm and the
groove and situated posterior to the Hensen’s
midgut. Keep in mind that the embryo was removed from the yolk
node, thus, section through the anterior region
for the fixation and sectioning. If the yolk was present in the section,
of the primitive streak will show the Hensen’s
it would fill the bottom part of the picture.
node, while more posteriorly the sections may pass through the primitive pit.
Midgut -
Floorless region of the gut
Primitive groove Sections cut more posterior to the primitive pit
Posterior to the anterior intestinal portal
will reveal a central furrow, the primitive groove.
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Section through the somite In 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. Somites Segmented mesodermal blocks -
Located on either side of the developing spinal cord
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Arise from the dorsal mesoderm
Nephrotome A stalk-like connection between somites and lateral plate mesoderm Form: pronephros (anteriorly), mesenchyme (posteriorly) Mesenchyme -> mesonephros and metanephros Syn: intermediate mesoderm, mesomere and nephromere Hypomere Region of the mesoderm distal to the nephrotome -
Subdivided into two layers: a)
Somatic mesoderm
b)
Splanchnic mesoderm
Section through the primitive streak In this centre of the picture, note the primitive streak which consists of the primitive groove and the two primitive ridges. Note the ectoderm, mesoderm and endoderm. Primitive streak Longitudinal cleft formed at the surface of amniote blastodisc by a convergence of cells Gastrulation begins as the epiblast cells migrate towards and into the streak. o
Hensen’s node
Thickened area at the anterior end of the primitive streak through which cells invaginate to form the notochord. It is homologous to the dorsal lip of the blastopore in amphibian embryos. Syn: Primitive knot
Primitive ridges Thickened margins on each side of the primitive groove.
