Toward the end of the second week after fertilization, transverse segmentation of the paraxial mesoderm begins, and it is converted into a series of well-defined, more or less cubical masses, also known as the somites, which occupy the entire length of the trunk on either side of the middle line from the occipital region of the head.
Contra o final da segunda semana despois da fecundación, empeza a segmentación transversa do mesoderma paraxial, e convértese nunha serie de masas ben definidas máis ou menos cúbicas, tamén chamadas somitas, que ocupan toda a lonxitude do tronco a cada lado da liña media desde a rexión occipital da cabeza.