About two weeks after fertilization, the cells begin to specialize. For the first time it has been possible to trace the process in a human embryo.
from
Hanno Charisius
One can ponder for a long time which is the most important developmental step in a person’s life: Birth? Puberty? Parenting? Old age and even death? Few of them will think of gastrulation. This process begins about two weeks after a sperm has fertilized an egg cell and takes about a week. During this time, a cluster of cells becomes something orderly. The cells also begin to specialize in the various tasks that they will take on in the organism. About half of all human embryos do not make it to this stage of development. Now, for the first time, a research group has succeeded in observing a human embryo at this all-important step.