The neural tube, which gives origin to the brain and the spinal cord, is one the first structures to appear during the embryonic development of mammals. In addition to the organs of the central nervous systems, the embryonic cells that form the ectoderm also differentiate to form other organs, such as the skin, including its glands and sense receptors; the cornea and other internal parts of the eye; the inner ear; and the epithelial lining of rectum and mouth.
Cells of the mesoderm originate the notochord, a primitive structure that eventually becomes the vertebral column. The mesoderm also originates muscles and bones; the organs of the circulatory system, such as heart, blood, arteries and veins; kidneys and urethra; and the dermis of the skin, the middle layer between the external epidermis and the internal hypodermis.
The endoderm forms various organs of the digestive system, including the stomach, intestines, liver and pancreas. Other organs originated from the endoderm include the bladder, and some organs of the respiratory system such as parts the trachea, lungs and pharynx. Some glands, such as the thyroid, parathyroid and thymus, which plays an important role in body immune defense, are also endoderm-originated.
Although the three embryonic cell types that form the ectoderm, endoderm and mesoderm always originate the same organs in all species of mammals, this process can occur in different ways. Marsupials, like kangaroos, have short gestational periods and initiate their organ formation much earlier than other mammals. In most species, the beginning of organ development depends on the length of the gestational period.