• Pre-embryonic period (first 2 weeks)
  -- includes cleavage, implantation
  and gastrulation.
• Embryonic period (3rd - 8th week)
  -- includes induction.
• Fetal period (3rd to 9th month)
  -- includes growth and development.

• Begins 1.5 days after fertilization.
• Completed in 5-6 days, before implantation.
• Zygote becomes a hollow blastula.
• Late blastula composed of:
  -- inner cell mass
  -- trophoblast layer

• Trophoblast cells "digest" their way into the uterine lining.
• Trophoblast cells will become the chorion.
• Urine tests for pregnancy detect hCG (human chorionic gonadotrophin).

• Formation of the three primary germ layers
• Epiblast migrates inward through the primitive streak to form both mesodern and endoderm.

• Begins with 2-layered embryo.
• Hypoblast (lower layer) grows down
  around "yolk" to form yolk sac.
• Epiblast (upper layer)
  -- grows up to form amnionic sac.
  --invaginates through primitive streak
  to form mesoderm and endoderm.

• Basic structures of all organ systems are completed during this phase (3-8th week)
• Induction of organ systems from interaction of three primary germ layers.
• Tubular "heart" beats at 3 weeks.
• Embryo the size of a pea at 5 weeks.

• Three grow out of the embryo.
• The fourth (the chorion) surrounds but has no contact with the embryo. It is formed from the trophoblast layer.

Extraembryonic membranes:
Chicken

• Yolk sac = encloses yolk; has blood vessels for food transport to embryo.
• Amnion = holds fluid around embryo.
• Allantois = stores wastes;
  blood vessels for gas exchange.
• Chorion = gas exchange.

• Yolk sac = not functional
• Amnion = "bag of waters" around embryo
• Allantois = its blood vessels used in umbilical cord.
• Chorion = from trophoblast; forms placenta with allantois; secretes hCG; boundary in placenta between mother and fetus.

• The interaction of any two primary germ layers.
• For example, formation of the spinal cord.

• Mesoderm (specifically, the notochord) induces the overlying ectoderm to form the neural tube, which becomes the spinal cord and brain.
• This is primary induction because the interaction is between primary germ layers.

• If closure of neural tube is incomplete,
  child is born with exposed neural tissue.

• The interaction of two cell layers derived from the same primary germ layer.
• For example, formation of eye.

• Optic cup (outgrowth of brain) induces overlying ectoderm to become lens of eye.
• Optic cup becomes retina, its lip the iris.
• This is secondary induction because both the optic cup and the skin are from ectoderm.

• For example, the structural similarities of early embryos of vertebrates.

• Does "ontogeny recapitulate phylogeny"?
• Does embryonic development retrace the path of evolutionary history?
• If not, then why do human embryos have gill pouches?

• The pouches never function as gills.
• Gill pouches serve as intermediates in the development of other structures.

• 1st pair become Eustachian tubes from throat to middle ear behind eardrum.
• 2nd pair of pouches becomes the tonsils.
• 3rd pair becomes thymus gland.
• 4th pair the parathyraoids (Ca++ levels)

• Third to Ninth month.
• Growth (from 1 ounce to 7.5 pounds).
• Head to body ratio decreases.

• 12 wk = swallowing & sucking reflexes.
• 16 wk = fills womb, abdomen expands.
• 17 wk = "quickening" (abortion deadline
  according to English Common Law).
• 23 wk = can survive with intensive care.
• 40 wk (+ 2) is considered full term.

• Baby (7.5)
• Amnionic fluid (2) & placenta (1.5)
• Uterus (3) & breasts (1)
• Blood (3) and body fluid (2.5)
• "Unnecessary" (not really) (7.5)

• 1st trimester = States cannot outlaw abortion in first 3 months of pregnancy.
• 2nd trimester = States may prohibit abortions except those for health reasons.
• 3rd trimester = States may prohibit abortions all together.

• 1st = dilation of cervix (3 mm to 10 cm)
  = 8-20 hours for 1st baby (3-8 later)
  = breathing to avoid premature push
• 2nd = delivery of baby (0.3-1.5 hr)
  including crowning, episiotomy, and
  cutting umbilical cord.
• 3rd = delivery of placenta (5-10 min later)

• Breathing & relaxation to break
  cycle of pain-tension-pain
• Father is involved.