1st week: Spermatozoa wriggle through the cumulus mass releasing acrosomal enzymes on contact. This allows the sperm to reach the oocyte. The oocyte then becomes impenetrable to further sperm.

Note the proliferation of cells that become an invasive syncytiotrophoblast, drawing the embryo into the endometrium of the uterine wall. The future amniotic cavity is beginning to form in the center of the epiblast (blue cells)."

4th week: Neurulation - initial development of the neural folds and future nervous system.

Embryonic folding transforming the embryo from a flat disc to a three-dimensional vertebrate body form.

Initial septation of the atria of the heart.

Development of the separate ventricles and major arteries leaving the heart.

Conversion of circulation from fetal to air-breathing pattern before birth.

Conversion of circulation from fetal to air-breathing pattern after birth.

Rotation of the stomach as it develops.

Origin of the human face from the fusion of five primordia or "facial swellings."