Gastrulation
Amphioxus
• In Amphioxus gastrulation begins by the flattening of the
blastula, loss of the blastocoel, and formation of the
archenteron - the embryonic gut cavity that is lined with
endoderm. After flattening, two cell layers can be distinguished
- ectoderm and endoderm.
• chordamesoderm, or the longitudinal mid-dorsal group of
mesoderm cells, moves into the roof of the archenteron during
gastrulation and gives rise to the notochord
• after flattening, the process of folding continues to further form
the archenteron as well as the blastopore, or external opening of
the gastrula
• further differentiation of cells occurs through the process of
budding off of mesodermal cells to:
- form pouches that will later become organs
- within these pouches are spaces that will become the body
cavity or coelom
- notochord formation proceeds with the condensing of the
chordamesoderm into the notochord
- the neural tube then forms from pinching of the ectoderm
over the notochord
Gastrulation in amphibians
•In amphibian gastrulation is changed slightly due to the larger
amount of yolk contained within the egg - the cells at the animal
pole are retained for the formation of the embryo while the yolkfilled cells at the vegetal pole are used more by the embryo as an
energy source
• gastrulation is initiated by invagination of cells to form the
dorsal lip of the blastopore
• cell movements cause the pushing out of the ectoderm and
inward movement of the endoderm and yolk-filled cells
• the archenteron is formed and the blastocoel is slowly filled
with cells and lost
• the yolk-filled cells of the vegetal pole remain for a short time
to fill the space between the dorsal and ventral lip of the
blastopore, thus forming the yolk plug
• the mesoderm gradually differentiates from the rest of the cells
of the gastrula, as do the chordamesoderm cells which go on to
form the notochord
• the gastrula then progresses into the neural tube formation
stage, called neurulation
Gastrulation in birds

with birds,
we begin to see the meroblastic type of development of the
embryos, significantly more yolk than in the previous
examples, and the movement of cells is different as the
cells lie more in sheets rather than in a ball.
• two processes lead to cell movement in the chick embryo:
- delamination: sheets of cells split into separate layers
- ingression: individual surface cells migrate to the
interior of the embryo
• during delamination, two layers of cells form (the
hypoblast and the epiblast) with a cavity in between that is
comparable to the blastocoel in amphibians - separation of
these two layers results in the formation of two regions of
the blastodisk, the area opaca and the area pellucida
• during ingression the primitive streak forms (a
longitudinal thickening of cells along the blastoderm of
large-yolked eggs) through which prospective
chordamesoderm and mesoderm cells move inward - cells
of the hypoblast are replaced by endodermal cells
• the primitive streak lengthens along the surface of the
yolk through ingression - the embryo grows longer and
occupies more of the area pellucida
• after gastrulation, the process of neurulation, or formation
of the neural tube and associated structures, occurs