THE NERVOUS SYSTEM. 



493 



of neurocytoplasm (dendrites) is significant. When the cell-proliferation near 

 the lumen has ceased, the supply of new cells ceases, and as the cells of the 

 inner layer continue to differentiate into cells of the mantle layer, the inner 

 layer, being no longer replenished from within, is reduced to the single layer of 

 cells which remain behind as ependyma cells (p. 488). 



Differentiation of the Peripheral Neurones of Cord and 

 Epichordal Segmental Brain. 



Efferent Peripheral Neurones. The differentiation of a mantle or 

 neurone layer from the outer part of the original nuclear layer is practically 

 universal throughout the whole neural tube. It appears first and is conse- 

 quently most advanced, however, in the ventral part of the lateral walls of the 

 cord and epichordal brain. The axones of neuroblasts occupying the basal plate 

 of this region of the neural tube grow out through the external limiting mem- 



FIG. 426. Ventral part of wall of lumbar cord of 7o-hour duck embryo, showing efferent root 



fibers first emerging from cord (combined from two sections) . Cajal. 

 A, Spinal cord; B, perimedullary space; C, meningeal membrane; a, b, cones of radially directed 



axones; c, d, cones of transversely directed axones; D, bifurcated cone; E, F, cones crossing 



perimedullary space; C, aberrant cones. 



brane and emerge as the efferent ventral root fibers. The appearance of these 

 early root fibers in the duck is shown in Fig. 426. The process is similar in 

 the human embryo and begins about the third week. The neurones thus 

 differentiated are the efferent peripheral neurones. 



In some forms, at least, cells appear to migrate out from the tube along with 

 the efferent root fibers. Their fate is not certain, but they probably either 

 metamorphose into the neurilemma cells or possibly form part of the sympa- 

 thetic ganglia (see p. 499) . In general the questions affecting the differentiation 



