Here you will see a mix of slides and schematics to give you a feel for the anatomy
of the embryonic disc of chickens. The embryonic disc is the mass of cells that
sits on the surface of the yolk and gives rise to the chick and its extra-embryonic
membranes. The embryonic disc is also called the blastoderm, the blastodisc, the
disc of blastomeres or, at later stages, the blastula. The embryonic disc arose
by meroblastic cleavage of the germinal disc. Recall that the portion of a hen's
egg that we call the "yolk" in everyday terms is really a single cell, the female
germ cell (or the oocyte or the ovum). It is termed "megalecithal" because of the
huge quantity of yolk. Cleavage is incomplete (meroblastic) and is restricted
to the small portion of yolk-free cytoplasm called the germinal disc.
Diagramatic Views of the Embryonic Disc
This series of diagrams shows the details of the embryonic disc you will see in
the slides below. The first diagram shows the embryonic disc as it sits on the
yolk and the plane of section of the slides you will be viewing. The following
diagrams show the sectioned embryonic disc and some of the important structures
you should know and be able to identify on the slides.
| The Whole Yolk and Disc: |
 |
Here you see the yolk isolated from the chicken
egg. The embryonic disc is seen as the dark structure on the top. The plane
going through the yolk and embryonic disc shows the plane of section seen
in the following diagrams. |
| The Sectioned Yolk and Disc: |
 |
Here you see the sectioned yolk and embryonic
disc. Some details of the anatomy of the embryonic disc are visible and
will be discussed in the next diagram. At this stage, the embryonic disc
could also be called the blastula. |
| The Enlarged Sectioned Disc: |
 |
From this enlarged cross-sectional view of the chick blastula, some important
anatomical details are visible and labelled:
- Ao = Area opaca
- Ap = Area pellucida
- 1 = Blastocoel
- 2 = Subgerminal space
|
Sectional Views of the Chick Blastula
 |
| This is a cross section of the chick blastula. The area
pellucida (Ap) and areas opaca (Ao) are visible, as are the subgerminal
cavity (1) and the blastocoel (2). The epiblast is seen as the continuous
layer along the top, while only fragments of the hypoblast and yolk are
seen below. The fragments of hypoblast are the precursors to the continuous
hypoblast which will form later and fragments of yolk are just that - bits
of the existing yolk caught in the section. Click on this
picture or scroll down to see selected areas enlarged. |
Selected Enlarged
Areas of the Chick Blastula
The two images below correspond with the numbered boxes in this picture. |
1 |
2 |
| This higher magnification shows
a portion of the area pellucida. Note the cells of the hypoblast (arrows)
and the epiblast above. The blastocoel (1) is visible, as is a portion of
the yolk (dark area, bottom left). |
This higher magnification shows
a portion of the area opaca. Note the epiblast (1), the yolk granules (2)
and hypoblast (3). |
Return
to the Cleavage Index
View
Gastrulation in the Megalethical Embryo
Proceed
to the Gastrulation Index
Department of Integrative Biology, University of Guelph, Guelph, Ontario,
Canada N1G 2W1.
