1. Embryology
2. Embryonic Period
3. Fetal Period

1. the study of the origin and development of a single individual
2. the first 8 weeks
3. the remaining 30 weeks

1. the zygote (fertilized oocyte) moves toward uterus
2. blastomeres - daughter cells are formed creating the 4-cell
3. morula - solid cluster of 12-16 blastomeres
4. early blastocyst - around 60 cells
5. late blastocyst - implants to uterus

1. the inner cell mass divides into 2 sheets, the epiblast and the hypoblast
2. the amniotic sac - formed by an extension of the epiblast (outer = amnion, inner = amniotic sac cavity), filled with amniotic fluid
3. the yolk sac - formed by an extension of the hypoblast, digestive tube forms but not a major source for embryo, gives rise to earliest blood cells and vessels
4. these combined are called the bilaminar embryonic disc

1. primitive streak forms on the surface of the epiblast; forms the 3 primary germ layers
2. endoderm - formed from migrating cells that replace the hypoblast
3. mesoderm - formed between the epiblast and endoderm
4. ectoderm - formed from epiblast cells that remain

1. the notochord forms from primitive node and ectoderm (notochord is site of future vertebral column)
2. neurulation - ectoderm starts forming brain and spinal cord
3. neural crest - forms sensory nerve cells
4. somites - mesoderm forms our first body segments

1. "tadpole shape" 
2. embryo folds to create primitive gut (site of future digestive tube and respiratory structures)
3. ectoderm >>> epidermis, brain, spinal chord, neural crest
4. endoderm >>> inner epithelial lining and glands of digestive, respiratory and urinary tracts
5. mesoderm >>> everything else

1. limb buds form
2. embryo first looks recognizably human
3. head is disproportionately large
4. all major organs in place

• time of maturation and rapid growth
• cells are differentiating during the first half
• normal births occur 38 weeks after conception
• premature birth is one before 38 weeks

Measurements of the Embryo / Fetus

1. at 8 weeks - 3 cm
2. at 12 weeks - 9 cm
3. at 16 weeks - 14 cm
4. at 20 weeks - 19 cm
5. at 30 weeks - 28 cm
6. at 38 weeks - 36 cm  (and 6 - 10lbs)