HISTOGENESIS OF THE NERVOUS TISSUES 305 



bridges of cells that later disappear. In the hind-brain region, certain 

 ganglia of the cerebral nerves develop from the crest but are not segment- 

 ally arranged. 



The cells of the spinal ganglia differentiate into: (i) ganglion cells, 

 and (2) supporting cells, groups which are comparable to the neuroblasts 

 and spongioblasts of the neural tube. The neuroblasts of the ganglia 

 become fusiform and develop a primary process at either pole ; thus these 

 neurons are of the bipolar type (Fig. 307, d). The centrally directed 

 processes of the ganglion cells converge, and, by elongation, form the 

 dorsal roots. They penetrate the dorso-lateral wall of the neural tube, 

 bifurcate, and course cranially and caudally in the marginal layer of the 

 spinal cord. By means of branched processes they come in contact with 

 the neurons of the mantle layer. The peripheral processes of the gang- 

 lion cells, as the dorsal spinal roots, join the ventral roots, and together 

 with them, constitute the trunks of the spinal nerves (Fig. 325). 



At first bipolar, (Fig. 310, A), the majority of the ganglion cells be- 

 come unipolar, either by the fusion of the two primary processes or by the 

 bifurcation of a single process. The process of the unipolar ganglion is 

 now T-shaped. Many of the bipolar ganglion cells persist in the adult, 

 while others develop several secondary processes and thus become multi- 

 polar in form. In addition to forming the spinal ganglion cells, neuro- 

 blasts of the ganglion crest are believed to migrate ventrally and form the 

 sympathetic ganglia (Fig. 325). 



Differentiation of the Supporting Cells of the Ganglia.- The support- 

 ing cells of the. spinal ganglia at first form a syncytium in the meshes of 

 which are found the neuroblasts. They differentiate: (i) into flattened 

 capsule cells, which form capsules about the ganglion cells; and (2) into 

 sheath cells, which ensheath the axon processes of both dorsal and ventral 

 root fibers and are continuous with the capsules of the ganglion. It is 

 probable that many of the sheath cells migrate peripherally along with 

 the developing nerve fibers (Harrison). They are at first spindle-shaped, 

 and, as primary sheaths, enclose bundles of nerve fibers. Later, by the 

 proliferation of the sheath cells, the bundles are separated into single fibers, 

 each with its sheath (of Schwann), or neurilemma. Each sheath cell forms 

 a segment of the neurilemma, the limits of contiguous sheath cells being 

 indicated by constrictions, the nodes of Ranvier. 



The Myelin Sheath. During the fourth month an inner myelin, or 

 medullary sheath appears about many nerve fibers. This consists of 

 a spongy framework of neurokeratin in the interstices of which a fatty sub- 

 stance, myelin, is deposited. The origin of the myelin sheath is in doubt. 

 By some (Ranvier) it is believed to be a differentiation of the neurilemma, 

 the myelin being deposited in the substance of the nucleated sheath cell. 



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