Page  Cell - Model # C1 & 2 A&P
 Numbers correspond to embryo Model Numbers

9. Day 12
     e. Amnion - Still considered a Bilaminar Embryo the amniotic cavity enlarges. It's floor is the 
         epiblast of the bilaminar embryo 
     f. Primary Yolk Sac - it's roof is the hypoblast of the bilaminar embryo 
     g. These spaces will soon coalesce to become the Chorionic Cavity.  The light gray represents
     2. Syncytiotrophoblast - most of the darker gray (outermost) area
10. Day 14
     g. Chorionic Cavity 
     h. Secondary Yolk Sac - a new yolk sac develops from the hypoblast and displaces the primary.
     2. Syncytiotrophoblast - continue to invade the uterus and begin developing the placenta 
     3. Connecting Stalk will soon become the umbilical cord
     4. Prochordal Plate - this will become the cephalic region of the embryo
Embryonic Disc - Trilaminar Embryo - NO MODEL AVAILABLE
- referred to as gastrula in lower animals - Day 15-16 
- this event is marked by the development of Ectoderm, Mesoderm, and Endoderm, all from the Epiblast. 
- migration of Mesenchyme cells (embryonic connective tissues from mesoderm) between 
  ectoderm and endoderm.  
11. Human Embryo - approximately 28-30 somites or 28 days - about 5 mm long
     The first 3 Pharyngeal Arches are now visible (eventually there will be 5) - these are visible in all 
     vertebrate embryos.  Long ago referred to as "gill slits", because in fish they give rise to the gills 
     and other structures, however, in mammals they give rise to: the auditory ossicles, much of the mandible, 
        nerves of the neck, the hyoid bone, the thymus, etc. 
      - therefore, damage to the arch in the early fetus will be reflected as defects in the structures they become
     - The Somites (segmented structures composed of mesoderm) - by week 5 there will be 42-44 pairs of somites
     - somites will develop into: vertebrae, muscles, dermis, cartilage, etc.
12. Human Embryo - approximately or 32 days - between week 4-5 - about 7 mm long (1 inch is about 15 mm long)
      - note that the limb buds are clearly visible.  
      g. Chorionic Cavity - note the umbilical vessels (2-arteries are blue, 1-vein are red - why these colors ?)
      5. Chorion 
      6. Chorionic Villi - the chorion will send out finger-like structures called Villi into the uterus.  
          These villi contain embryonic blood vessels and are surrounded by the maternal-blood-
          containing lacunae mentioned above.  -  i.e. the villi are bathed by maternal blood.
- Neural Groove (Day 18), soon to become the Neural Tube forms in the dorsum of the embryo & 
   becomes the Nervous System
- Day 25 - closure of the anterior portion (to become the forebrain or cerebral cortex) 
- Day 27 - closure of the posterior portion (to become the spinal cord)
- Anencephally and Spina Bifida are Neural Tube Defects (NTD's) due to failure of 
   the tube to close as seen with Folic Acid deficiency - indicated by increased Alpha Fetal Protein.
  1. (mandibular) - becomes malleus, incus, facial skeleton, muscles of mastication, Mandibular branch of CN V
      - lining of pouch becomes ext. aud. meatus, middle ear cavity, Eust.Tube, tymp.membrane
  2. (hyoid) - becomes stapes, parts of hyoid bone, styloid process, muscles of facial expression, Facial N
      - lining of pouch becomes palatine tonsil
  3. parts of hyoid bone, Glossopharyngeal N
      - lining of pouch becomes thymus, thyroid gland (& part of parathyroids) 
  5. absent or rudimentary - no structures, however, the lining of its pouch gives rise to the 
     parafollicular cells of the thyroid gland which produce calcitonin
  4. &6. laryngeal cartilages and muscles, branches of Vagus N
     - lining of pouch # 4 becomes part of parathyroids
- by the middle of week 3, mesenchymal cells aggregate in an area known as the cardiogenic area
- Day 21 - 2 parallel tubes lie side-by-side 
- Day 22 - two tubes above fuse into one - Contractions begin
- by the end of week 4 - blood flow is unidirectional - no longer ebb & flow.
- by middle of week 4 the atrial and ventricular partitioning begins and finishes by week 5.
- interventricular foramen closes by week 7
- Probe Patent Foramen Ovale - in about 25% people, a probe can be passed through the fossa 
  ovale - because the septum primum is large enough to prevent "by-pass" this is not 
  a pathological condition