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Beginning of week 1,
The union of ovum and sperm forms a zygote -
End of week 1,
The zygote has become a blastocyst and is embedded in the prepared endometrium -
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 -
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) -
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 -
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 -
Primitive muscle cells from mesoderm in mandibular arch differentiate
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Upper lip begins formation (maxillary processes fuse with medial nasal processes)
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Fusion of medial nasal and maxillary processes to form upper lip is completed
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Wrist and elbows can flex and eyes begin to develop pigment
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Third to Ninth Month
