THE FROG 79 



nucleated cells which flatten cut and form the neurilemma; 

 the white substance appears at a comparatively late period. 



Draw a portion of a nerve fiber, showing as many as pos- 

 sible of the points described above. The nerve fiber should 

 be represented at least one centimeter in diameter. 



2. Cross-section of the spinal cord. Observe the median 

 dorsal and ventral longitudinal fissures. The ventral 

 margin may usually be distinguished from the dorsal by 

 means of an artery that runs along the ventral longitudinal 

 fissure. Sometimes this artery is removed in making the 

 preparation; in this case the ventral margin must be deter- 

 mined by observing the positions of the ventral white com- 

 missure and the larger cell bodies (see below). Observe 

 the central canal with its epithelial lining (ependyma). 

 The central portion of the cord is composed of gray matter 

 consisting mainly of nerve cells (or more strictly speaking 

 the cell bodies of nerve cells) and nerve fibers; the outer 

 portion or white matter contains no cell bodies of nerve 

 cells but consists chiefly of fibers which extend in a longitu- 

 dinal direction. With respect to the distribution of struc- 

 tures on each side of the dorso-ventral median line, the cross- 

 section of the cord shows marked bilateral symmetry. 



Many fibers cross over from one side of the cord to the 

 other. Ventral to the gray matter is a rather conspicuous 

 pair of bundles of fibers crossing each other at various 

 angles, forming the ventral white commissure. A narrow 

 median tract of less conspicuous fibers crossing in the gray 

 matter above the central canal forms the dorsal gray com- 

 missure; a similar tract of fibers crossing in the gray matter 

 below the central canal forms the ventral gray commis- 

 sure. 



