The embryogenesis is the passage in the service life after the fertilization - the development of the embryo, from zygote (fertilized ovum). The organisms can differ drastically in how the embryo is developed, mainly when belong to different edges. For example, the embryonic development of the placental mammals begins with division of the zygote in eight cells uncommited, that soon to form a ball (morula). The external cells turned trophectoderm or trophoblast, that forms in combination with the uterine mother of the endometrial weave the placenta, necessary to feed fetal through the maternal blood, whereas the inner cells turned the internal mass cellular which they will form all the organs of the fetus (the bridge between these two parts possibly form the umbilical cord). However, the fly of the fruit zygote first forms of syncytium in form of sausage, that continues being a cellular one, but with nuclei of cells.

Patterns it is important to determine that the cells are developed in which the organs. This is half-full by the signaling between adjacent cells by proteins in its surface, and by molecule gradients of secret signaling. An example is the retinoic acid, that forms a gradient in the head to the direction of the tail in the animals. The retinoic acid enters the cells and activates the Hox genes in a dependent form of the concentration - the Hox genes differ in the amount of retinoic acid that they require for the activation and therefore are to the limits differentials rostral expression, of colinear way with their genomic order. As Hox genes codifies transcription factors, this causes that different combinations from both activated Hox and other genes in discreet segments anteroposterior cross-sectional of the neural tube (neuromeros) and patterns related in surrounding weaves, such as branchial arcs, lateral mesoderm, the neural crest, the skin and the endoderm, in the head to the direction of the tail.