The three germ layers ectoderm, mesoderm, and endoderm give rise to their own tissues and organs during the embryonic period, which occurs from the third to the eighth week of development. The mesodermal layer contains paraxial, lateral plate, and intermediate mesoderm. Paraxial mesoderm gives rise to mesenchyme of the head that is organized into somites, which give rise to sclerotome, dermatome, and myotome. These somites form all of the supporting tissues of the body. The vascular and urogenital systems as well as the spleen and cortex of the suprarenal glands are all derived from mesoderm. Homeobox genes regulate segmentation and axis formation, playing a role in cranial-to-caudal patterning of the derivatives of all three germ layers.