24. Embryonic Period: Weeks 7 and 8 and External Embryo Appearance

  1. Week 7 of development
    1. CONNECTION BETWEEN THE GUT AND YOLK SAC consists of only the small yolk stalk
      1. Umbilical herniation occurs; intestines during rotation of the gut enter the extraembryonic coelom in the proximal portion of the umbilical cord
    2. THE LIMBS change markedly during this week
      1. The forelimbs project over the heart
      2. Notches are seen between the rays in the hand plates indicating future fingers
  2. Week 8 of development: this is the final week of the embryonic period
    1. THE FINGERS are noticeably webbed and short
    2. NOTCHES now are seen between the toe rays, and the tail bud is still visible
    3. THE LIMB REGIONS are clear, fingers lengthen, toes are distinct, and the tail bud disappears by the end of this week
    4. THE EMBRYO has human characteristics, but the head is distinctly large (about one-half of embryo)
      1. The neck region is established, and the eyelids are obvious
      2. The abdomen is less bulging, and the umbilical cord is reduced in size
      3. The intestine is still within the proximal portion of the umbilical cord
    5. THE EYES usually open, but near the end of week 8 the eyelids begin to meet and fuse
    6. THE EXTERNAL EARS (AURICLES) assume their final shape but are still low set
    7. THE EXTERNAL GENITALIA are not distinct enough for accurate sexual identification
  3. External embryo appearance
    1. BY THE END OF WEEK 4 the embryo has about 28 somites, the ventral body wall has closed, and the major external features are somites and pharyngeal arches. The age of the embryo now is often expressed in somites or as the crown-rump (CR) length (sitting height) in millimeters. The standing height or crown-heel (CH) length is sometimes used for 8-week and older specimens, but these are hard to make accurately
      1. The CR length is measured from the skull vertex to the midpoint between the apices of the buttocks. Variations in flexion result only in approximate real age measurements
        1. 5 weeks: 5-8 mm; 6 weeks: l0-14 mm; 7 weeks: l7-22 mm; 8 weeks: 28-30 mm
    2. THE EXTERNAL APPEARANCE during month 2 changes due to the great size of the head and formation of the limbs, ears, nose, and eyes. By the beginning of week 5, the fore- and hind-limbs appear as paddle-shaped buds
    3. AGE ESTIMATION usually relies on 2 commonly used references, namely, the onset of the last menstrual period (LMP) and the time of fertilization
      1. Since the zygote does not form until the second week after the onset of the last normal menstrual period, 14 ? 2 days are deducted from the so-called menstrual age to get the actual or fertilization age of the embryo
      2. Foot-length correlates with CR length and is used in aging an incomplete or macerated fetus
      3. Fetal weight is not too accurate for use in age determination in light of any maternal metabolic disturbance

embryonic period: weeks 7 and 8 and external embryo appearance: image #1