Central Nervous System
Immediately after conception a multicellular blastula is formed, with three cell types: ectoderm, mesoderm, and endoderm. Bones and voluntary muscles will subsequently develop from mesodermal cells, and intestinal organs will develop from endodermal cells. The ectoderm will develop into the nervous system, skin, hair, eye lenses, and the inner ears. Two to 3 weeks after conception the neural plate develops on the dorsal side of the embryo, starting as an oval thickening within the ectoderm. The neural plate gradually elongates, with its sides rising and folding inward. Thus the neural groove is formed, developing eventually, when the folds merge, into the neural tube. By the end of the 4th week, three bubbles may be seen at the anterior end of the tube: the forebrain (prosencephalon), the midbrain (mesencephalon), and the hindbrain (rhombencephalon). The rest of the tube is elongated further and, keeping the same diameter, becomes the spinal cord (medulla spinalis). The forebrain will eventually become the cerebral cortex (cortex cerebri). During the 5th week the forebrain is divided into the diencephalon and the telencephalon. At the same time the hindbrain is divided into the metencephalon and myelencephalon. In approximately the 7th gestation week the telencephalon is transformed into cerebral hemispheres, the diencephalon into the thalamus and related structures, while the metencephalon develops
into the cerebellum and the pons, and the myelencephalon becomes the medulla (medulla oblongata).