Third to Eighth Week:The Embryonic Period ((organogenesis II))
Derivatives of the endoderm:• Formation of the epithelial lining of the gut tube;
• Folding of the embryonic disc results in the formation of the gut tube.
• Cephalocaudal folding caused by the rapidly elongated neural tube
• Lateral folding caused by the rapid growth of the paraxial mesoderm forming the somites.
• The gut tube:
• Foregut
• Hindgut
• Midgut connected with yolk sac (the vitelline duct.)
Derivatives of the endoderm:
Derivatives of the endoderm:
Derivatives of the endoderm:
• 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 gutBy 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 Month
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 crown-rump
length (CRL) and expressed in millimeters
External Appearance During the Second Month
increase in head size and formation of the limbs, face, ears, nose, and eyesBy 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 Month
