I think it is helpful for zoology students

1. The process of gastrulation in chick is peculiar as the egg is telolecithal and no true
archenteron being formed. Although chick’s gastrulation is very complicated and a
prolonged one, still it gets completed by the second day of incubation.
There exists a lot of discrepencies regarding the range of the process of gastrulation in
chick. Ede (1978) regarded the beginning of gastrulation after the formation of area
opaca and area pellucida. Balinsky (1981), Carlson (1996), were of the opinion that the
formation of hypoblast to be a pregastrular morphogenetic movement.
Here, we consider the formation of epiblast and hypoblast to be the starting point of
gastrulation, in accordance with that of Gilbert (2000).

6. The second step in gastrulation is the formation of primitive streak.
At the posterior region of area pellucida in the mid dorsal line primitive streak will appear as a
thickened area. It starts eight hours after incubation.
The thickening is because convergence of cells of blastoderm towards the centre. Usually in the
early stages the primitive streak is short and broad. It is called primitive streak.
It gradually extends to the middle of blastoderm.
At eighteen to nineteen hours of incubation, primitive streak is well developed. It is calleddefinite
primitive streak.
Along the middle line of primitive streak a narrow furrow is develop called Primitive groove. The edges
of groove are thick. They are called primitive folds.
At the anterior end of groove a mass of closely packed cells will be present. It is called "hensen's node"
or primitive knob.

7. In the centre of this node a pit is present. It is called primitive pit.
It represents the vestige of neurenteric canal.
The primitive streak elongates along with this, area pellucida will also elongates.
As the primitive streaks growing the cells from this region will invigilate into space
between epiblast and hypoblast This process is called immigration.
The immigrated cells will become prechordal plate, notochord, and mesoderm. Anterior
to primitive streak the mesoderm cells will not migrate to primitive streak.
This mesoderm free area is called proamnion. At this place head will develop.

13. The hypoblast cells get confined at the anterior portion of the area pellucida, at a region
(edge of area opaca) called the germinal crescent. This germinal crescent does not form any
embryonic parts, but it contains the precursors of the germ cells, that later migrate via the
blood vessels to the gonads.
The next cells to migrate into the blastocoel through Hensen’s node are the embryonic
mesodermal cells, like head mesenchyme and. the precordal plate mesoderm, that also
move anteriorly and position itself between the endoderm and the epiblast.
These early-ingressing, anteriorly moving cells push up the anterior midline region of 1 the
epiblast to form the head process .Thus, in chick embryo the head process forms anterior to
Hensen’s node.
The presumptive notochordal cells are the next to migrate through the Hensen’s node and
they extend up to the presumptive mid brain area where they meet the precordal plate
mesoderm. These notochordal cells form the hind brain and trunk at the level of Hensen’s
node.

14. Migration of Endoderm and Mesoderm Cells through the Primitive Streak:
In chick, gastrulation takes place by the coordinated passage of individual cells from the
exterior to the interior of the embryo by ingression. As soon as the primitive streak is
formed the epiblast cells start to migrate through it and into the blastocoel.
The primitive streak has a population of cells that are continuously changing, as the cells
converging on the primitive streak moves inward, their place is taken up by another batch
of cells.
The first group of cells to migrate through the Hansen’s node are the presumptive
endodermal cells that are destined to form the foregut. These cells pass down into the
blastocoel, migrate anteriorly and eventually displace the hypoblast cells