1078
UNIT 5
Continuity
28
Somite
Intermediate
mesoderm
Lateral plate
mesoderm
Coelom
Neural
crest
Neural
groove
Neural
fold
Neural
crest
Somite
Surface
ectoderm
Neural
tube
Notochord
Kidney and gonads
(intermediate
mesoderm)
Gut lining
(endoderm)
Parietal serosa
Visceral serosa
• Smooth
muscle of gut
• Limb
bud
Lateral plate
mesoderm
• Der
mis
Epidermis
(ectoderm)
Der
matome
Myotome
Sclerotome
Peritoneal cavity
(coelom)
Neural tube
(ectoderm)
Somite
(a) 17 days.
The flat three-layered
embryo has completed
gastrulation. Notochord and
neural plate are present.
(c) 22 days.
The neural folds have
closed, forming the neural tube
which has detached from the
surface ectoderm and lies
between the surface ectoderm
and the notochord. Embryonic
body is beginning to undercut.
(b) 20 days.
The neural folds form
by folding of the neural plate, which
then deepens, producing the
neural groove. Three mesodermal
aggregates form on each side of
the notochord (somite,
intermediate mesoderm, and
lateral plate mesoderm).
(d) End of week 4.
Embryo
undercutting is complete. Somites
have subdivided into sclerotome,
myotome, and dermatome, which
form the vertebrae, skeletal
muscles, and dermis respectively.
Body coelom present.
Amnion
Head
Tail
Left
Cut
edge of
amnion
Right
Primitive
streak
Amniotic cavity
Neural plate
Ectoderm
Mesoderm
Notochord
Endoderm
Yolk sac
Figure 28.12
Neurulation and early mesodermal differentiation.
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