SPINAL CORD. 429 



mic tracts. The gray matter in the dorsal zone develops later. 

 It becomes transformed into the posterior columna; its neuroblasts 

 form largely intrinsic neurones, their processes remaining in the 

 spinal cord, others give rise to the posterior cerebello-spinal 

 tract which ascends through the marginal velum to the vermis 

 superior of the cerebellum. Axones from the spinal ganglia, 

 forming the posterior roots of the spinal nerves, grow into the dor- 

 sal zone; and each axone divides T-like into a descending and an 

 ascending branch, and also gives off. many collaterals both before 

 and after division. The ascending fibers for a time form the 

 oval bundle of His on the surface of the tube ; later, they constitute 

 the marginal tract (Lissaueri) and the fasciculus cuneatus (Burd- 

 achi) and fasciculus gracilis (Golli). 



The third, or ependymal layer, lines the ventricle. It is com- 

 posed of neuroglia covered internally by columnar ciliated cells, 

 which appear about the fifth week. 



The Longitudinal Tracts (Figs. 101, 102 and 103). The white- 

 columns of the spinal cord are formed in the superficial, or neu- 

 roglia layer. At first they are composed of nonmedullated fibers, 

 and it is a remarkable fact that the various tracts receive their 

 myelin sheaths at definite periods between the fifth and ninth 

 months of intrauterine life (Flechsig). The anterior nerve- 

 roots and the anterior and lateral fasciculi proprii, and the pos- 

 terior nerve-roots and the oval bundles of His are first to make 

 their appearance. The axones of these nerves-roots and tracts 

 may be seen at the sixth week. The ascending cerebello-spinal 

 tracts follow, and the descending cerebello-spinal and the pyram- 

 idal tracts are last to appear; they are not developed before the 

 fourth or fifth month. According to Kahler, as quoted by Cun- 

 ningham, the tracts of the spinal cord are medullated as follows : 



1. Fibers of the fasciculus cuneatus with the posterior nerve- 

 roots, preceded by the medullation of the anterior nerve-roots 

 (fifth month). 



2. Fibers of anterior and lateral fasciculi proprii (fifth and 

 sixth months). 



3. Fibers of fasciculus gracilis (sixth month). 



4. Posterior cerebello-spinal tract (seventh month). 



