518 



TEXT-BOOK OF EMBRYOLOGY. 



gray columns, but contains a disproportionately small amount of fibers. A 

 further and later change consists in a rolling inward, as it were, of the dorsal 

 gray column so that it becomes separated from the ventral gray column, and 

 that portion of it formerly facing dorsally comes to face more mesially, the roots 

 entering more dorsally. This change may be due partly to the development 

 of the intermediate plate which has in the meantime taken place. In this 

 plate axones of tautomeric cells have begun to form the limiting layer of the 

 lateral funiculus. From the cells of the intermediate plate are formed the 

 neck of the dorsal gray column, also the cells of Clarke's column and the 



Funiculus gracilis 



Floor plate - 



Vent. long, sulcus - 



- * -- - Dors, funiculus (cuneatus) 

 Dors, gray column 



Dors, root 



Marginal furrow 

 Intermed. plate 



- -\- — Cylinder furrow 



^Central canal - 



' — Lat. gray column 



- Ventro-lat. funiculus 

 • Vent, gray column 



- Vent, root 



Vent, funiculus 

 Vent. sp. artery 



Fig. 446.— Half of a transverse section of the spinal cord of a human fcetus of about 3 months. His. 



processus reticularis. In the course of these developments, the ventro-lateral 

 ground bundles, formed primarily by heteromeric and tautomeric cord cells, 

 receives various accessions. These are first the long descending inter- 

 segmental tracts from epichordal brain nuclei in the formatio reticularis 

 which as they proceed down the cord naturally overlap externally the ground 

 bundles already formed there. They include the medial longitudinal fasciculi, 

 tracts from Deiters' nuclei and the rubrospinal tracts which occupy the ventro- 

 lateral funiculi external to the ground bundles. In the lateral funiculi there 

 are also added the ascending tracts from cord nuclei to suprasegmental structures. 



