The Embryonic Rise of the Nervous System
In invertebrates generally the inward migration of the central nervous system comes about through a process of delamination, that is, the splitting off of a thickened band of cells from the hypodermis along the ventral side of the body.
In the case of most vertebrates, on the contrary, the central nervous system is buried by a process of invagination. In cyclostomes and in many teleosts, however, the embryonic foundation of the central nervous system is probably laid down first as a solid rod of cells which afterwards becomes hollowed out.
The first step in the formation of the vertebrate nervous system occurs toward the end of the gastrula stage when the ectoderm in front of the blastoporic lip thickens to form the neural or medullary plate, which extends lengthwise along the dorsal side of the elongated embryo. This structure is morphologically continuous and, since the embryonic ectoderm is not conspicuously given to metameric expression, it is never marked off into segments, as in the underlying mesoderm in the formation of somites.
The lateral edges of the long flat medullary plate next become elevated, thus transforming it into a troughlike canal, the medullary groove. Continuing their growth upward, the edges eventually meet and grow together, forming the medullary tube. The cavity within this embryonic tube is the forerunner of the ventricles of the brain and the central canal of the spinal cord. The closure of the tube begins in the middle region and extends both ways, so that for a time both ends still remain open.
At the posterior end there may exist, in addition to the trough-like opening to the outside, a temporary inward passage-way around into the primitive enteric cavity through the blastopore. This is the neurenteric canal, which remains open up to the fourth week in the human fetus, although the medullary groove begins to close about the fifteenth day. The anterior end of the medullary tube remains unclosed longer than the posterior end in the form of the so-called neuropore. Amphioxus retains the neuropore throughout life. Possibly in this forerunner of the vertebrates it may serve as some sort of a sense organ.
As the medullary tube closes there form on either side along the seam of fusion two distinct ridges of medullary tissue, the neural crests (Figs. 602 and 603). At first continuous with the tube, these crests later gain independence by the insertion of invading mesenchymal tissue between them and the tube itself. Eventually the crests break up into chains, forming both the sensory dorsal ganglia of the spinal nerves and the ganglia of the autonomic system, which are of motor function. When the neurons of the dorsal ganglia sprout they send their dendrites into the various organs of the body while their neurites establish a secondary connection into the cord. The autonomic ganglia also becomes connected with the cord, but by neurites which grow out from cells within the cord itself. Similar relationships are established in the brain region.
All of the nervous tissues of the body are derived, either directly or indirectly, from the medullary tube, with the exception of cells and fibers that contribute to the olfactory epithelium.