THE NERVOUS SYSTEIM 



451 



lieteromeric cells (Fig. 317,1 and 2). The axones of a few cross in the 

 dorsal white commissure. In a few others the axone divides, one 

 branch going to the white matter of the same side, the other to the 

 white matter of the opposite side — hecateromeric cells (Fig. 317, 4). 



The axones of many of these cells are short, constituting the short 

 fibre tracts (fundamental columns — ground bundles) of the cord (see 



VeniraX 



Fig. 317. — Transverse Section through Spinal Cord of Embryo Chick of Seven 

 Days Incubation (Golgi Preparation). The Ijoundary between the future gray and 

 white matter is indicated by the dotted line, i and 2, Heteromeric column cells whose 

 axones {ax) pass through the anterior commissure to the opposite ventral white columns; 

 3, tautomeric column cells whose axone (ax) passes to the ventro-lateral white column 

 on the same side; 4, hecateromeric column cell whose axone (ax) bifurcates, one branch 

 entering the ventral white column of the same side and the other branch passing 

 through the anterior commissure to the opposite side. A number of longitudinal 

 fibres in the white columns are shown, cut transversely or obliquely. 



page 474; others form long ascending fibre tracts, passing up through 

 the cord to the brain (see page 467.) Terminals and collateral 

 branches of these longitudinal axones, especially of the short ones, 

 are constantly re-entering the gray matter to end in arborizations 

 around the nerve cells (Fig. 318). 



