502 TEXT-BOOK OF EMBRYOLOGY. 



in which their axones develop. This is probably true in a general way, but the development 

 of both axones and sheaths requires further study before any law can be exactly formulated. 

 Coarse fibers apparently become medullated early^ the sheaths of such fibers being usually 



thicker. 



Although the myelin sheath is apparently an accessory structure, its formation is of 

 great importance, not only from the above reason, but also because its appearance possibly 

 indicates the assumption by the neurone of its capacity for the precise performance of its 

 final functions. The functional significance of the myelin sheath is not, however, entirely 

 clear. Its importance is enhanced by the fact that its integrity depends upon the integrity 

 of its neurone and that we possess precise stains for demonstrating both its normal and 

 abnormal conditions. 



In the region of the rhombencephalon, the neural crest very early exhibits 

 a division into three masses : a glossopharyngeo-vago-accessorius, an acustico- 

 fac talis, and a trigeminus. These masses soon become separated from each 

 other and from the neural tube, the glossopharyngeus also showing a partial 

 separation from the vago-accessorius mass (Fig. 434). 



The vago-accessorius group, at about three weeks, is a mass of cells much 

 larger at the cranial end and continuous by a narrow band of irregular cells 

 with the spinal neural crest. The cranial end of the mass shows a partial 

 division into a dorsal and ventral part. The former becomes the ganglion of 

 the vagus root, the latter the ganglion of the trunk (nodosum). The glosso- 

 pharyngeus mass likewise shows a division into a dorsal group of cells, the 

 future ganglion of the root and a ventral group, the future ganglion of the 

 trunk (petrosum). The two ventral groups are associated with epidermal 

 thickenings (placodes), but it is doubtful whether any ganglion cells are 

 derived from the thickenings. These thickenings probably represent the 

 uiiciienings associated in water-inhabiting Vertebrates with the development of 

 certain sense organs, either lateral line or epibranchial (see p. 459). At this 

 stage there are no afferent fibers, the cells not yet being differentiated into 

 neurones. Some fibers found among the cells are efferent (see p. 495). The 

 glossopharyngeus cells lie in the region of the third branchial arch, the vagus 

 in the region of the fourth. 



During the fourth and fifth weeks the processes of the cells begin to develop 

 (Fig. 434), and the cell masses finally become definite ganglia with afferent root 

 fibers passing into the neural tube and peripheral processes passing outward, 

 forming, with the associated efferent fibers, the peripheral branches of the nerves 

 in question (Fig. 435). The root and trunk ganglia of the vagus and glosso- 

 pharyngeus, respectively, are also now connected by fiber bundles instead of 

 cellular strands. At the same time there is a diminution of cells in the caudal 

 part of the vago-accessorius group, this part finally being composed almost ex- 

 clusively of efferent fibers emerging from the lateral surface of the medulla and 

 cord. A few groups of cells (accessory root ganglia) persist, however, and develop 



