The epiblast and hypoblast migrate to form the primitive groove and ridges along the primitive streak. At the cephalic end, a thickening called Henson's node forms. Cells from Henson's node push into the midline to form the notochord beneath the ectoderm. Somites form from the dorsal mesoderm of Henson's node and organize bilaterally. The lateral plate mesoderm splits into somatic and splanchnic mesoderm, forming the coelom cavity between them. By 25-26 hours, somites appear as compact cell masses lateral to the neural folds.

1. Mesoderm and
Notochord
formation
10 Dec

2. •The epiblast and hypoblast continue to
migrate and form a groove along the
length of primitive streak called primitive
groove. It has thick margins which are
named as called as Primitive Ridges.
•At cephalic end (Cephalic refers to the
head of the embryo, while caudal refers
to the tail) of primitive streak a thick
pack of cells form a local thickening
called as Henson’s Node.

4. •NOTOCHORD FORMATION:
• After primitive streak and endoderm is well
established, cells began to push in from the region
of Henson’s node to form the rod like NOTOCHORD
in the midline beneath the ectoderm.
• An embryo incubated for 18-20 hours, it is seen that
ectoderm has spread and become organized into a
coherent layers of cells merging peripherally with
the Yolk and the marginal areas where the
expanding germ layers merge with the underlying
yolk is known as Germ Wall and the cavity b/w the
yolk and the endoderm which has been called
Gastro-coel is now termed as Primitive Gut.

5. •From Henson’s Node, dorsal mesoderm is formed and
is organized into SOMITES (The somites are outdated
term: primitive segments) are a set of bilaterally paired blocks of
paraxial mesoderm that form in the embryonic stage of somitogenesis,
along the head-to-tail axis in segmented animals)
•The lateral plate mesoderm is splitted into two sheets
like layers viz Somatic mesoderm and
Splanchic Mesoderm, with a space b/w them.
•The cavity formed b/w somatic and splanchic
mesoderm is Coelom.
•Somites are seen in 25-26hr embryo, these are
compact cell masses lying immediately lateral to neural
folds.