THE NERVOUS SYSTEM. 519 



These are the dorsal spino-cerebellar tracts from Clarke's columns, ventral spino- 

 cerebellar tracts, and tracts to mid-brain roof and thalamus (spino-tectal and 

 thalamic}. Finally (fifth month) the descending tracts from the pallium are 

 added, the direct and crossed cor tico- spinal (pyramidal] tracts, the latter being 

 thrust, as it were, into the lateral funiculus. 



The development of the cord, then, is produced by (i) the proliferation of 

 the epithelial cells and the formation of the nuclear and marginal layers; (2) 

 the multiplication, differentiation and growth of the neuroblasts (mantle layer) ; 

 (3) the development of the ventral roots; (4) formation of the funiculi (white 

 columns when myelinated) by the growth into the marginal layer of (a) dorsal 

 root fibers of the cord, the ascending arms of which overlap those root fibres 

 entering higher cord segments, (b) cord neuroblasts forming intersegmental 

 (ground bundle) tracts next to the gray matter, (c) descending intersegmental 

 tracts from the segmental brain, representing continuations principally of cere- 

 bellar efferent tracts, (d) afferent suprasegmental tracts from cord nuclei, 

 (e) descending pallio-spinal tracts. In addition to this, there are general 

 factors of growth, such as increasing vascularization, increasing amount of 

 neurone cytoplasm (especially dendrites) , increased size of axones and, finally, 

 the acquisition by the latter of myelin sheaths. 



The vertebral column grows faster in length than the inclosed spinal cord. 

 The result of this is that the caudal spinal nerves making their exit through the 

 intervertebral foramina are, so to speak, dragged caudalward and instead of 

 proceeding outward at right angle to the cord, pass caudally to reach their 

 foramina. The leash of nerve roots thus formed, lying within the caudal part 

 of the vertebral column, constitutes the cauda equina. The coverings of the 

 cord retain their original connections at the caudal end of the vertebral canal 

 and form a prolongation of the cord membranes enclosing the thin, terminal 

 part of the cord, the jilum terminate. 



The Epichordal Segmental Brain. 



In the fifth week, the walls of the rhombencephalon are comparatively thin. 

 In the caudal region of the medulla oblongatta (p. 484), the dorsal part of each 

 lateral wall is upright and is bent at a considerable angle with the ventral 

 part (basal plate), the groove on the inner surface between the two being the 

 sulcus limitans. The roof of this region is formed by the thin expanded roof 

 plate (Figs. 436-439). 



Anterior to this, the roof plate is not expanded, the alar plates almost 

 meeting in the mid-dorsal line. This thicker part of the roof is the rudiment 

 of the cerebellum. Its caudal edges are attached to the expanded roof plate (see 

 P- 532). 



