THE NERVOUS SYSTEM. 



503 



neurones (p. 449). The migrating bodies of these neuroblasts are checked at 

 the inner boundary of the marginal layer, but their growing axones enter the 

 marginal layer and there, apparently on account of their inability to penetrate 

 the external limiting membrane, turn cranially or caudally, or bifurcate, and 

 form longitudinal ascending and descending fibers. These longitudinal fibers 

 constitute a part of the future white columns (see p. 507) , and their cells are 

 therefore often called column cells. Many axones from such cells in all parts 

 of the lateral walls (Jteleromeric or commissural column cells) pursue a ven- 

 tral course through the mantle layer, arching around near the periphery and 



FIG. 441. Part of a section through the lumbar spinal cord of a 76-hour chick embryo. Cajal. 

 A, Ventral root; 5, spinal ganglion; C, bifurcation of dorsal root fibers forming beginning of dorsal 

 funiculus; a, b, c, neuroblasts showing various stages of differentiation into intermediate 

 neurones, some, at least, (c) becoming heteromeric column cells; d, efferent neurone. 



crossing the floor plate, ventral to the lumen, to become longitudinal ascending 

 and descending fibers in the marginal zone of the opposite side. These early 

 decussating axones form, in the cord, the beginning of the anterior commissure 

 (Fig. 441). Other neuroblasts, the axones of which do not cross the median 

 line, become tautomeric column cells. 



It is about this time that the afferent root fibers enter the marginal layer of 

 the dorsal part (alar plate) of the lateral wall and form in the marginal layer 

 the various bundles of longitudinal fibers above described (dorsal funiculus, 

 tractus solitarius, descending vestibular, and spinal V) (Figs. 441, 442, 436, 437^ 



