MORPHOGENESIS OF CENTRAL NERVOUS SYSTEM 819 



Within the ventral gray matter of the spinal cord, fusiform bipolar cells arise 

 which send processes at intervals out into the marginal layers and from thence 

 outward through the external limiting membrane of the tube at the levels 

 corresponding to the developing dorsal root ganglia. The groups of processes 

 which thus emerge from the neural tube below a single dorsal root ganglion 

 soon unite with the ventrolateral processes of the dorsal root ganglion cells to 

 form the ventral root of the spinal nerve. Within the neural tube the cell 

 bodies of the ventral root fibers soon form multipolar neuron cells. 



As development proceeds, the cell bodies of the neurons within the dorsal 

 root ganglia become encased by capsular cells which develop from some of 

 the neural crest cells (fig. 352D). The capsular cells in consequence are of 

 ectodermal origin and they are continuous with the neurilemma sheath. The 

 cells of the neurilemma sheath also arise from certain neural crest cells and 

 from cells within the neural tube. These cells migrate distad as sheath cells 

 along with the growing nerve fiber. The neurilemma or sheath of Schwann 

 arises as an outward growth from the cytoplasm of the sheath cells; the 

 neurilemma sheath thus appears in the form of a delicate tube surrounding 

 the nerve fiber (axis cylinder) of the neuron (352D). Later on, a secondary 

 substance appears between the nerve fiber (axis cylinder) and the neurilemma 

 in many nerve fibers. This substance is of a fatty nature and forms the myelin 

 (medullary) sheath (fig. 352E). Myelin deposition by sheath cells depends 

 primarily upon an axis cylinder stimulus and not upon the sheath cells, for it 

 is only a particular type of nerve fiber, the myelin-emergent fiber, which pos- 

 sesses the ability to form myelin (Speidel, '33). In the peripheral nerve fibers, 

 the neurilemma at certain intervals dips inward toward the axis cylinder, 

 forming the node of Ranvier. The area between two nodes is known as an 

 internodal segment (fig. 352B). One sheath cell is present in each internodal 

 segment. The nerve fibers of the peripheral nervous system with respect to 



Fig. 355 — Continued 



arrows drawn in the ventral portions of the nerve tube indicate the ventro-medial ex- 

 pansion of lateral portions of the developing nerve tube with the subsequent formation 

 of the ventral median fissure. In E the dorsal, ventral, and lateral columns or funiculi 

 of white matter are shown. (F) Diagram depicting some of the principal fiber tracts 

 of the spinal cord of man. Ascending tracts on the right; descending tracts on the left. 

 (Redrawn from Ranson, 1939. For reference see G.) (G) Ventral view of human 

 spinal cord, nerves removed, showing cervical and lumbar enlargements. (Redrawn from 

 Ranson, 1939, The Anatomy of the Nervous System. Philadelphia, Saunders.) (H) 

 Diagram revealing the relation of the meninges, i.e., the protective membranes of the 

 central nervous system, to the spinal cord. (Redrawn from Ranson, 1939. For reference 

 see G.) (I) Schematic diagram of transverse section through myelencephalon (medulla), 

 portraying dorso-ventral position of nuclei of origin in motor plate and the nuclei of 

 termination in alar plate of cranial nerves associated with the myelencephalon. 



