Day 1: Conception
1. Ovulation
2. Fertilization
Day 2-3: Zygote develops into Morula
1. Morula: round mass of cells
2. Morula passes through fallopian tube
Day 4: Morula develops into blastocyst
1. Blastocyst: hollow sphere
  1. Inner Cell mass: mound on inner surface
  2. Primitive trophoblast: outer shell of sphere
    1. Together with mesoderm will form Chorion
2. Blastocyst floats free in uterus
Day 7-14
1. Embryo Embeds into wall of uterus
Day 8: Bilaminar Germ Disk develops
1. Ectoderm (Epiblast) lines amniotic sac
  1. Skin and appendages
  2. Nervous system
    1. Adrenal medulla
    2. Anterior pituitary
    3. Salivary Glands
2. Mesoderm forms between Ectoderm and Entoderm
  1. Bone, Muscle, Cartilage, and connective tissue
  2. Serous linings
  3. Cardiovascular system
  4. Kidneys
  5. Genitalia
3. Entoderm (Hypoblast) lines Yolk Sac
  1. Gastrointestinal tract
  2. Liver
  3. Gall Bladder and biliary tract
  4. Pancreas
  5. Respiratory tract
  6. Gonadal germ cells
Day 9-13: Placenta develops
1. Trophoblast forms with lacunae
2. Maternal and Trophoblast Vessels develop
3. Uteroplacental circulation begins

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This page was written by Scott Moses, MD. This page was last revised before 5/10/08, prior to consistent tracking of revision date. This page was last published on 6/1/2008. Please send comments and questions to the editor. Contact information may be found on the page About the Family Practice Notebook. These pages are best viewed with Microsoft Internet Explorer 6.0.