Two cell stage (called morula) reached approximately 30 hours after fertilization.

Each blastomere; equal cytoplasmic and chromosome number.

Morula after spending about 3 days in uterine tube enters uterine cavity through narrow uterine ostium on 4th day in the 16-64 cell stage.

Central cell of morula known inner cell mass (form embryo).

Peripheral cell called outer cell mass (form protective and nutritive membranes of embryo).