Week 4 (CRL = 4 mm)
By the fourth week after conception the shape of the embryo changes. Laterally, it now resembles a C-shaped cylinder, where it folds at the head and tail. A “tail” is apparent during the embryonic period because the brain and the spinal cord develop rapidly than other systems. The tail disappears as the rest of the body catches up with growth of the central nervous system.
- CNS: The neural tube closes at each end. The cranial end of the neural tube will form the brain while the caudal end will form the spinal cord.
- SENSORY: As an outgrowth of the forebrain, eye development initiates. The nose begins as two pits and the inner ear starts to develop from the hindbrain.
- CARDIO and RESPI: The heart begins partitioning into four chambers and begins beating. Blood is circulating through the embryonic vessels and chorionic villi. The trachea starts to develop as a bud on the upper gut and branches into the two bronchial buds.
- DIGESTIVE: Primitive gut development takes place as the embryo folds laterally. The stomach development is initiated as a widening of the tube-shaped primitive gut occurs. The liver, gallbladder and biliary ducts begin as a bud from primitive gut.
- GENITOURINARY: the primordial germ cells are present on embryonic yolk sac.
- MUSCULOSKELETAL: The upper limb buds are already present at this time and they look like flippers. The lower limb buds appear at this period.
- INTEGUMENTARY: The mammary ridges that will develop into mammary glands starts to appear.
Week 6 (CRL = 13 mm)
- NERVOUS: Pituitary glands and cranial nerves are developed. The head is flexed sharply because of the rapid brain growth.
- SENSORY: Development of the eyelid is already beginning at this time. The external ear development begins in the neck region as six swellings.
- CARDIO and RESPI: Formation of blood is primarily on the liver. The partition of the heart into four chambers is completed at this point. Three right and two lung lobes develop as outgrowths of the right and the left bronchi.
- DIGESTIVE: The liver and the kidney occupy most of the abdominal cavity that results to the containment of most intestines within the umbilical cord. At this period, the stomach is nearing its final form and the development of the upper and lower jaw is observed.
- GENITOURINARY: The kidneys, which are located near the bladder in the pelvis, are occupying much of the abdominal cavity. Primordial cells are now incorporating to the developing gonads. At this point, the male and female gonads are still undifferentiated and are identical in appearance.
- MUSCULOSKELETAL: The arms are paddle-shaped and the fingers are webbed. Development of the toes and the feet occurs in a similar pattern however, it occurs a few days later than the hands and the arms. Bones are cartilaginous at this point but the ossification of the skull has already started.
- INTEGUMENTARY: The development of the mammary glands is initiated. Also starting at this time is the development of the tooth buds for primary or so-called deciduous teeth.