THE NERVOUS SYSTEM. 



497 



and form a ridge on the dorsal surface of the neural tube, this ridge being 

 known as the neural crest (Fig. 429). 



In the spinal cord, at three weeks, the neural crest has separated from the 

 cord and split into two longitudinal bands. The ventral border of each band 

 shows a transverse segmentation into rounded clumps of cells, forming the 

 rudiments of the spinal ganglia which later become completely separated. The 

 efferent roots have begun to develop but the afferent roots appear later (fourth 

 week, Fig. 434). The cells composing these rudiments are polyhedral 

 or oval rather than columnar and proliferation still proceeds among them 

 A differentiation of these cells soon begins. Some, usually larger cells 



Fig. 430. — Part of a transverse section through the cord and spinal ganglion of a 56-hour chick 



embryo (combined from two sections) . Cajal. 

 A, Efferent cell of dorsal root; B, cone of growth of central process (afferent dorsal root fiber) of 



spinal ganglion cell; C, bifurcation of afferent root fibers in cord, forming beginning of dorsal 



funiculus or dorsal white column of cord. 



begin to assume a bipolar shape. Their central processes grow toward the 

 dorsal part of the lateral walls (alar plate) of the neural tube which they enter 

 (Fig. 430) , becoming afferent (dorsal) root fibers. These fibers enter the mar- 

 ginal layer and there divide (Figs. 430 and 441) into ascending and descend- 

 ing longitudinal arms which constitute the beginning of the dorsal (posterior) 

 funiculus of the cord. The peripheral processes of the developing ganglion 

 cells grow toward the periphery, uniting with the ventral root and forming 

 with it the various branches of the peripheral spinal nerve (compare Figs. 

 263, 265, 432 and 404). Other peripheral branches pass as a part of the 

 white ramus communicans to the sympathetic ganglia through which they 



