SPINAL CORD REGENERATION. I 481 
portions of the cord and their origin from neuroblasts situated 
behind the cut ends may be observed in some cases. None of 
the nerves arise from nerve cells at the cut surfaces. The con- 
nective tissue fibers, identifiable by their granular appearance, 
are found in large numbers in the middle of the gap. Toward 
the anterior end of the cord they have left the position among 
the nerve fibers occupied by them slightly further back and lie 
entirely outside the bundle. Their place is taken by a number of 
very fine, branched processes which arise from the epithelial 
cells of the canalis centralis of the anterior stump. For. some 
little distance m front of the actual end of the cord, these cells 
are found in large numbers lying on all sides of the central canal. 
Frequent mitotic figures are to be seen among them. ‘The cells 
situated anterior to this mass and those near the tip of the stump 
show processes of the type noted above. ‘Those from the cells 
nearest the end of the cord extend, in this stage, a short distance 
into the fibrous bundles. 
Though the mitoses found among the epithelial cells of the 
canalis centralis are not sufficient to account for their extra- 
ordinary number, it is apparent that they are rapidly proliferat- 
ing. Examination of the position of the spinal ganglia and the 
sensory roots in relation to the cord reveals that, following the 
retraction of the cord, a progression of its anterior end has 
occurred. In one of these embryos, VIII 65, the center of the 
last spinal ganglion of the anterior segment lies 131.25 » behind 
the point of entrance of its sensory root into the cord. As in 
normal embryos at this stage the same relative measurement 
rarely exceeds 90 to 95 u, it is evident that the cord has been 
pulled forward to some extent since the sensory root became 
attached to it. On the other hand, the end of the anterior 
piece of the cord is 304 uw behind the point of entrance of this 
sensory root. As the greatest distance between the end of the 
cord and the entrance of the sensory root of the next cephalic 
ganglion found in any of the embryos in this or preceding stages 
measures only 131 uy, it is further evident that the spinal cord has 
increased somewhat in length since the 4 day period. The mass 
of epithelial cells of the central canal thus seems to designate the 
