The zygote carries the genetic material provided by the sperm and the egg. Once formed the zygote will divide many times by mitosis until it gives rise to a new individual. Like this, all cells that make up an individual's body have the same genetic heritage that existed in the zygote.
Nevertheless, throughout embryonic development cells undergo a process of cell differentiation in which some genes are "on" and others are "off", and only the "activated" coordinate the functions of the cells.
Thus arise cell types with distinct shapes and functions, which are organized into tissues. Assembled tissue assemblies form the organs. Organ groups form the systems that in turn form the organism.
Cells - Tissues - Organs - Systems - Organisms
The science that studies this process of individual development from the zygote is embryology.
Stages of Embryonic Development
The animals present a great diversity of embryonic development, but generally, in almost all occur three consecutive phases: segmentation, gastrulation and organogenesis.
At segmentationEven as the number of cells increases, there is virtually no increase in the total volume of the embryo, as cell divisions are very rapid and cells have no time to grow.
In the next phase, which is the gastrulation, the increase in cell number is accompanied by the increase in total volume. In this phase, the cellular differentiation begins, occurring the formation of germinal leaflets or embryonic leaflets, which will give rise to the tissues of the individual.
In the next stage, which is the organogenesis, the differentiation of the organs occurs.
Let's look at each of these phases for animals in general and then comment on human embryonic development.