430 EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



5. Ascending anterior cerebello-spinal and spino-thalamic tracts 



(?) (eighth month). 



6. Pyramidal tracts (ninth month). 



Fissures (Fig. 101). The so-called posterior median fissure 

 is in reality a septum of neuroglia. At no time is it a true fissure. 

 Its mode of formation is still in doubt. It appears to be pro- 

 duced by thickening in the roof-plate due to the elongation of 

 ependymal cells. From the sixth week this neuroglia septum ex- 

 tends ventrally until it reaches the center of the cord. 



The posterior lateral sulcus is the groove between the lateral 

 border of the posterior column and the dorsal border of the 

 lateral column. It is the development of the posterior and lateral 

 columns that produces the fissure. The embryonic central fis- 

 sure is obliterated and the postero-lateral surface of the cord ren- 

 dered prominent by the formation of the lateral fasciculus proprius, 

 the cerebello-spinal and the pyramidal tracts. 



Anterior Median Fissure. That is a true fissure. It begins 

 to be formed at the sixth week, when the anterior columnae and 

 the earliest fibers of the anterior fasciculus proprius are developing. 

 It deepens with the growth of the gray columnae and of the 

 anterior fasciculus proprius and its walls are further heightened, 

 in the fourth or fifth month, by the descent of the anterior pyr- 

 amidal tracts. In this manner there is produced a bulging of the 

 anterior surface on either side of the median line, which increases 

 with the medullation of the longitudinal tracts up to the ninth 

 month. The ridges thus produced, failing to fuse completely, 

 become the walls of the anterior median fissure. The partial 

 fusion which does occur between the two ridges is due to the for- 

 mation of the white anterior commissure of the cord. 



