Running the whole dorsal length of the embryo from the pole of the animal hemisphere to the blastopore, the outer layer of cells forms a long groove. Next, the edges of the groove fuse forming the neural tube. Finally, the tube becomes detached from the outer layer of cells and, eventually, differentiates into the brain and spinal chord of the fetus.
The gastrula has two initial cell layers, the outer ectoderm and the inner endoderm. A third cell layer or mesoderm develops in between the ectoderm and endoderm. The mesoderm arises from cells located around the blastopore where the ectoderm and endoderm meet. The cells of the ectoderm give rise to the epidermis and to the tissues derived from the epidermis such as hair, nails, many glands, the eye lenses, and the linings of the mouth, nasal cavity, and anus. The endoderm develops into the lining of the digestive tract and those structure derived from the digestive tract such as the liver, the pancreas, the thyroid, the bladder, and the respiratory passages and lungs. The mesoderm gives rise to many of the interstitial tissues such as muscle, connective tissue (including blood and bone), and the notochord. The one major interstitial tissue group not derived from the mesoderm is the nervous system. Nervous system tissue, derived from the neural tube formed during neurulation, arises from the ectoderm.
