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Preimplantation: Fertilization
Beginning of week 1,
The union of ovum and sperm forms a zygote -
Preimplantation: Implantation
End of week 1,
The zygote has become a blastocyst and is embedded in the prepared endometrium -
Embryonic Period - Week 2
Blastocyst grows by proliferation with differentiation,
Embryonic layers are formed,
Bilaminar embryonic disc is developed (superior layer is epiblast and inferior layer is hypoblast),
Placenta formation -
Embryonic Period - Beginning of Week 3
Primitive streak forms in bilaminar disc,
Some cells from epiblast layer migrate toward hypoblast layer and become mesoderm and endoderm,
Thickened into trilaminar embryonic disc,
Epiblast is now ectoderm,
Disc now has cephalic end (oropharyngeal membrane forms) and caudal end (cloacal membrane forms) -
Embryonic Period - End of Week 3
CNS begins to develop in embryo,
Specialized group of cells differentiates from ectoderm and is now neuroectoderm (localized to neural plate),
Formation of neural groove which deepens and is surrounded by neural folds,
Neural crest cells develop and join mesoderm to form mesenchyme -
Embryonic Period - Week 4
Trilaminar embryonic disc undergoes embryonic folding,
Future digestive tract tube is formed,
Neural folds undergo fusion and neural tube is formed,
Face and neck begin to develop,
Primitive mouth is now stomodeum,
Mandibular and maxillary processes appear,
Frontonasal process form,
Lens, optic and nasal placodes form -
Embryonic Period - Week 5
Primitive muscle cells from mesoderm in mandibular arch differentiate -
Embryonic Period - Week 6
Upper lip begins formation (maxillary processes fuse with medial nasal processes) -
Embryonic Period - Week 7
Fusion of medial nasal and maxillary processes to form upper lip is completed -
Embryonic Period - Week 8
Wrist and elbows can flex and eyes begin to develop pigment -
Fetal Period
Third to Ninth Month
