. Comparative anatomy. Anatomy, Comparative. THE NERVOUS SYSTEM 559 and become concentrated into two parallel strands dorsal to the spinal cord. The crest is continuous throughout the entire length of the central nervous system, except for an interruption in the head region between the fifth and seventh nerves. As development proceeds, each strand of neural crest cells migrates en masse ventrad. In the trunk region the cells become more and more concentrated and metamerically arranged as a series of ganglionic masses median to the myotomes. For each myotome, there develops both a somatic motor
. Comparative anatomy. Anatomy, Comparative. THE NERVOUS SYSTEM 559 and become concentrated into two parallel strands dorsal to the spinal cord. The crest is continuous throughout the entire length of the central nervous system, except for an interruption in the head region between the fifth and seventh nerves. As development proceeds, each strand of neural crest cells migrates en masse ventrad. In the trunk region the cells become more and more concentrated and metamerically arranged as a series of ganglionic masses median to the myotomes. For each myotome, there develops both a somatic motor root and a dorsal sensory ganglion. Within the sensory ganglia, two kinds of cells become differen- tiated. Toward the center, most of the cells become nerve cells, from each of which processes grow in two directions, one process into the dorsal. A B Fig. 461.—A, diagram of early spinal cord; B, later, showing increase in size and consequent ventral fissure, c. central canal; e, ectoderm; /, floor plate; g, anlage of spinal ganglion; nc, neural crest; r, roof plate; 5, sulcus limitans; v, ventral fissure. (From Kingsley's "Comparative Anatomy of ;) column of the tube and the other ventrally to join the somatic motor nerves which have already formed. The resulting mixed nerve gives off dorsal and ventral rami, to skin and muscles. The relations of the fibers of the sensory roots have already been described. Most of the peripheral cells of the ganglion become neurolemma cells. Possibly all neurolemma cells of sensory nerves are so derived. The development of the neural crest in the head region differs in some details from that of the spinal nerves just described. Olfactory nerve fibers, except those in the nervus terminalis, develop not from the neural crest but from the olfactory epithelium; and, since the optic nerve is strictly a fiber tract of the brain, its development also is not from the neural crest. The oculomotor nerve is somatic motor, like the
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