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Third to Eighth Week:
The Embryonic Period
((organogenesis II))
Derivatives of the endoderm:
1. Formation of the epithelial lining of the gut
Folding of the embryonic disc results in the formation of the
gut tube.
Cephalocaudal folding caused by the rapidly elongated neural
Lateral folding caused by the rapid growth of the paraxial
mesoderm forming the somites.
The gut tube:
Midgut connected with yolk sac (the vitelline duct.)
Derivatives of the endoderm:
Derivatives of the endoderm:
Derivatives of the endoderm:
2. Formation of the epithelial
ling of the allantois;
Embryonic foldings lead to the
incorporation of allantois in the
embryonic body
Futhrer development, the allantois
epithelia will form the epithelia of
the U.B and the urethra.
Derivatives of the endoderm:
3. Formation epithelial lining of the respiratory tract;
the primordium of the respiratory tract buds from the
foregut tub.
4. The tissues of the thyroid and parathyroid glands that
are formed as buds from the gut tube.
5. The liver tissues; formed by budding from the gut.
6. The thymus.
7. The epithelia of the tympanic cavity of the middle ear
and the Eustachain Auditary Tube.
Derivatives of the endoderm:
• In the fourth week, the buccopharyngeal memb
ruptures, establishing an open connection between the
amniotic cavity and the primitive gut
• By the fifth week, the yolk sac duct, allantois, and
umbilical vessels are restricted to the region of the
umbilical ring.
• In the seventh week the cloacal membrane breaks
down to create the opening for the anus.
• In the second month of development, the yolk sac lies
in the chorionic cavity
External Appearance During the Second
• 1-At the end of the fourth week, when the
embryo has approximately 28 somites,
the main external features are the somites and
pharyngeal arches
• second month the age of the embryo is then
indicated as the
length (CRL) and expressed in millimeters
External Appearance During the Second
• increase in head size and formation of the limbs,
face, ears, nose, and eyes
• By the beginning of the fifth week, forelimbs and
hind limbs appear as paddle-shaped buds
• four radial grooves(rays ) separating five slightly
thicker areas appear on the distal portion of the
buds, foreshadowing formation of the digits
External Appearance During the Second