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

1. Zygote - a fertilized oocyte - note that the sperm (male) pronuclei has not yet fused with the 
    oocyte (female) pronuclei.
    - the oocyte is fertilized in the distal fallopian tube and is in Metaphase II of Meiosis
    - fertilization can occur during day 14 of a 28 day reproductive cycle.  The oocyte can only be 
      fertilized for a very short period of time (as little as 15 hours after ovulation)
    - Zona Pellucida - NOT SHOWN ON MODELS - glycoprotein shell surrounding the embryo until the 
        blastula stage.  The human embryo must "hatch" out of this "shell" in order to implant (nidate).  
    - Zona Radiata  
    - Male and Female Pronuclei

2. Two-cell embryo - occurs approximately 30 hrs after fert - each cell is a Blastomere

3. Four-cell embryo

4. Eight-cell embryo - (actually, I only count 7, but call it 8)

5. Morula - about 16 blastomeres and occurs around day 3 after fertilization.
    - blastomeres still totipotent - each has the potential to develop into a complete individual, 
      identical twins or "clones" 

6. Blastula/blastocyst - hollow ball of cells - begins around day 4 (the time the embryo reaches the uterus)
     a. Inner Cell Mass - will become the embryo proper
     b. Trophoblasts - outer wall of the embryo and will contribute to the placenta & produce hCG (human chorionic 
	gonadotropin) which keeps the corpus luteum functioning (it produces progesterone to maintain the uterus for 
	pregnancy)
     c. Blastocoel - this is the fluid filled space
     - implantation or nidation will occur around day 6 after fertilization at blastula stage

7. Bilaminar Blastula - Day 7, the Inner Cell Mass differentiates into two layers.  
     a.  Epiblast (yellow)
     b.  Trophoblasts 
     c.  Blastocoele - will be lined by mesoderm to become the Primary Yolk Sac
     d.  Hypoblast - will give rise to yolk sacs when its cells line the blastocoel

8. Late Blastula - around day 8
     b. Trophoblasts - differentiate into:
         1. Cytotrophoblasts - surround the entire embryo and give rise to #2 below
         2. Syncytiotrophoblasts - "Syncytio" refers to the fact that these are Multinucleated.
              They are aggressively invasive cells that digest into the uterus to establish placenta
         3. Lacunae - spaces eventually filled with Maternal blood which will bath the chorionic villi
     c. Blastocoele - once lined by cells from the Hypoblast becomes the Primary Yolk Sac
     e. Amnion - surrounding the amniotic space - between inner cell mass/epiblast & trophoblasts