Fertilized egg. Diploid cell (2n). Zygote undergoes first cleavage ~30 hours after fertilization. Totipotent — can form entire organism.
Embryonic development begins at the zygote — one totipotent cell. Cleavage (rapid mitotic divisions without growth) creates a morula, then a hollow blastocyst with an inner cell mass (source of embryonic stem cells) and trophoblast. Gastrulation establishes the three primary germ layers: ectoderm, mesoderm, endoderm. Morphogen gradients — pioneered by Turing, implemented by proteins like Bicoid and Nanos in Drosophila — provide positional information. Cells at different concentrations adopt different fates. Lewis Wolpert's French Flag model elegantly captures how thresholds in a continuous gradient generate discrete cell identities.