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E3-E6: Small intestine. Loss of ventral attachment of mesentery to body wall as the two canals of I-E coelom open to the extra-embryonic coelom (chorionic sac). In E5 and E6, vitelline duct of yolk sac stalk is cut close to its attachment to the yolk sac in the embryo-embryonic coelom (yolk sac removed).
E3-E4: Urogenital sinus receiving two lateral ducts - the mesonephric ducts. Note fusion of urogenital sinus endoderm and ectoderm - the cloacal membrane. Rectum.
DEVELOPMENT OF THE NERVOUS SYSTEM
E4: Spinal cord has enlarged ventral horns (motor neurons).
DEVELOPMENT OF THE MUSCULOSKELETAL SYSTEM
E4: Spinal nerves (part of future brachial plexus) directed towards the flexor side of the limb fold. Note also the section of the lumbosacral region with neural tube and hindlimb bulge.
