Answer
By the third week of gestation three germ layers can be differentiated in the embryo. These are the ectoderm ( outermost layer), the mesoderm and the endoderm ( innermost layer). The nervous system develops from the ectoderm. The differentiation and development of the nervous system begins with the laying down of a neural plate in dorsal medial aspect of the embryo. The next step is that the neural plate sinks and curves at its edges to form a neural groove. The curving of the lateral folds continue until they meet to form a neural tube. The fusion of the folds begin in the cephalic region of the embryo and proceed both caudally and rostrally; by the 26th day of embryonic development this fusion of the neural tube is complete. Some important CNS structures that derive from the lumen of the neural tube are the central canal of the spinal cord and the vesicles of the brain.
As embryonic development proceeds, some ectodermal cells arrange themselves along both sides of the neural tube, These cells form the neural crests which give rise to the following structures:
The arachnoid mater and the pia mater
Schwann cells
Most of the peripheral nervous system
Sensory and autonomic nerves and ganglia
The adrenal medulla
The neural tube first gives rise to three primary brain vesicles. These are the forebrain (prosencephalon), the midbrain ( mesencephalon), and the hindbrain (rhombencephalon).
By the fifth week of development, secondary brain vesicle develop from two of the primary vesicle.. The forebrain gives rise to the telencephalon and the diencephalon. Two hindbrain derivatives, the metencephalon, and the myelencephalon are also formed at this time. No secondary vesicles develop from the mesencephalon.
Work Step by Step
The five secondary brain vesicles give rise to the following structures:
The cerebral hemispheres develop as outgrowths of the telencephalon.
The optic vesicles--later to become retinas-- are formed from the diencephalon.
The metencephalon becomes the midbrain.
The pons, and the cerebellum develop from the metencephalon.
The medulla oblongata arises out of the myelencephalon.
